`html2sprockets:assets` is a quick way to add 3rd-party js/css to your rails app that is asset pipeline compatible; creates local gem in `vendor/gems` and apply them in your `Gemfile`

html2sprockets.rake

require 'fileutils'

namespace :html2sprockets do

  desc "Setup env for gem creation"
  task :env do
    @gemname = ENV.fetch('NAME')
    @gemdir = File.join(ENV.fetch('GEMDIR', 'vendor/gems'), @gemname)
    @asset_dst_path = File.join(@gemdir, ENV.fetch('DST', 'vendor/assets'))
    @asset_src_paths = ENV.fetch('SRC').each_line.collect(&:strip)
  end

  desc "Create a local, vendor gem and use it in Gemfile"
  task gemdir: :env do
    FileUtils.rm_rf(@gemdir) if File.exists?(@gemdir)
    {
      "Gemfile" => <<-EOM.strip_heredoc,
        source 'https://rubygems.org'
        gemspec
      EOM
      "README.md" => <<-EOM.strip_heredoc,
        This gem is auto-generated by html2sprockets.
      EOM
      "Rakefile" => <<-EOM.strip_heredoc,
        require "bundler/gem_tasks"
      EOM
      "lib/#{@gemname}.rb" => <<-EOM.strip_heredoc,
        module #{@gemname.classify}
          class Engine < ::Rails::Engine
            initializer '#{@gemname}.assets.precompile' do |app|
              Dir[root.join('vendor/assets/*').to_s].each do |dir|
                app.config.assets.paths << dir
                app.config.assets.precompile += Dir["\#{dir}/**/*.*"].
                  reject  { |path|
                    case path
                    when /\\/test\\//
                      # whitelist dir/subdir here
                      true
                    else
                      case File.basename(path)
                      when 'WHITELIST'
                        # whitelist filename here

                      when /^_/, /\.json$/, /^Gruntfile/
                        # blacklist filename here
                        true
                      end
                    end
                  }.
                  collect { |path|
                    path[dir.to_s.length+1..-1].
                      gsub(/.(sass|scss)$/, '.css').
                      gsub(/.(erb|coffee)$/, '')
                  }
              end
            end
          end
        end
      EOM
      "#{@gemname}.gemspec" => <<-EOM.strip_heredoc,
        $:.push File.expand_path("../lib", __FILE__)
        Gem::Specification.new do |s|
          s.name        = "#{@gemname}"
          s.version     = "0.0.1"
          s.authors     = ["html2sprockets"]
          s.summary     = "#{@gemname}"
          s.files       = Dir["{lib,vendor}/**/*", "README.md"]
          s.add_dependency "sprockets-rails", ">= 2.3.3"
        end
      EOM
    }.each do |path, content|
      relpath = File.join(@gemdir, path)
      FileUtils.mkdir_p(File.dirname(File.expand_path(relpath)))
      open(relpath, "wb") {|f| f.write(content) }
    end

    gemline = "gem '#{@gemname}', path: '#{@gemdir}'\n"
    lines = IO.read('Gemfile').each_line.to_a - [gemline] + [gemline]
    open('Gemfile', 'wb') {|f| f.write(lines.join) }
  end

  desc "Copy assets into gem's `vendor/assets`"
  task assets: :gemdir do
    @asset_src_paths.each do |source|
      seen = {}
      pattern_map = {
        "#{source}/**/*.js" => 'javascripts',
        "#{source}/**/*.{css,css.map,sass,scss}" => 'stylesheets',
        "#{source}/**/*.{eot,svg,ttf,woff,woff2}" => 'fonts',
        "#{source}/**/*.*" => 'images',
      }

      pattern_map.each do |pattern, subdir|
        Dir[pattern].each do |src|
          next if seen[src]
          next if File.directory?(src)
          seen[src] = true

          dst = File.join(@asset_dst_path, subdir, @gemname, File.basename(source), src[source.length+1..-1])
          FileUtils.mkdir_p(File.dirname(dst))
          if src =~ /\.css\b/ && (not src =~ /\.map$/)
            need_erb = false
            newdata = IO.read(src).gsub(/\burl\s*\((.+?)\)/) {|match|
              relpath   = match.gsub(/url\s*\(['"]?|['"]?\)/, '')
              if relpath =~ /^(\/\/|\w+:)/
                match # url?
              else
                need_erb = true
                abspath = File.expand_path(relpath, File.dirname(src))
                refsrc = @asset_src_paths.find {|s| abspath.index(s) == 0 }
                assetpath = File.join(@gemname, File.basename(refsrc), abspath[refsrc.length+1..-1])
                "url(<%= asset_path '#{assetpath}' %>)"
              end
            }
            dst = dst + (need_erb ? ".erb" : "")
            open(dst, 'wb') {|f| f.write(newdata) }
          else
            FileUtils.cp src, dst
          end
          puts dst
        end
      end
    end
  end
end

sample.bash

# download and unzip https://github.com/hakimel/reveal.js/archive/3.1.0.zip
rake html2sprockets:assets NAME=revealjs SRC="/Users/choonkeat/Downloads/reveal.js-3.1.0/css 
/Users/choonkeat/Downloads/reveal.js-3.1.0/js 
/Users/choonkeat/Downloads/reveal.js-3.1.0/lib 
/Users/choonkeat/Downloads/reveal.js-3.1.0/plugin"
# app/assets/stylesheets/reveal.css:/*
# app/assets/stylesheets/reveal.css: *= require revealjs/css/reveal.css
# app/assets/stylesheets/reveal.css: *= require revealjs/css/theme/black.css
# app/assets/stylesheets/reveal.css: *= require revealjs/lib/css/zenburn.css
# app/assets/stylesheets/reveal.css: */
# <%= javascript_include_tag "revealjs/lib/js/head.min.js", "revealjs/js/reveal.js" %>

# download and unzip https://github.com/twbs/bootstrap/releases/download/v3.3.5/bootstrap-3.3.5-dist.zip
rake html2sprockets:assets NAME=bootstrap SRC="/Users/choonkeat/Downloads/bootstrap-3.3.5-dist/css
/Users/choonkeat/Downloads/bootstrap-3.3.5-dist/fonts 
/Users/choonkeat/Downloads/bootstrap-3.3.5-dist/js"
# app/assets/stylesheets/bootstrap.css:/*
# app/assets/stylesheets/bootstrap.css: *= require bootstrap/css/bootstrap.min
# app/assets/stylesheets/bootstrap.css: */
# app/assets/javascripts/bootstrap.js://= require bootstrap/js/bootstrap.min

# download and unzip http://galleria.io/static/galleria-1.4.2.zip
rake html2sprockets:assets NAME=galleria SRC="/Users/choonkeat/Downloads/galleria-1.4.2/js 
/Users/choonkeat/Downloads/galleria-1.4.2/plugins 
/Users/choonkeat/Downloads/galleria-1.4.2/themes"
# app/assets/javascripts/galleria.js://= require galleria/js/galleria-1.4.2.min