gistfile1.txt

class AbsPathTag < Liquid::Tag
  def initialize(tag_name, src, tokens)
    super
    @src = src
    @src.strip!
  end
 
  def render(context)
    @base_url = context.registers[:site].config['url']
    absoluteize(@src)
  end

  def absoluteize txt
    @base_url + txt
  end
end

Liquid::Template.register_tag("abspath", AbsPathTag)