dump.rb

require 'jekyll'

site = Jekyll::Site.new(Jekyll.configuration)

array = Dir.glob('./_posts/*').map {|post|
  document = Jekyll::Document.new(post, site: site, collection: Jekyll::Collection.new(site, 'post'))
  document.read

  {
    title:      document.data['title'],
    slug:       document.data['slug'],
    body:       document.content.gsub(/##/, '###'),
    markup:     'redcarpet',
    type:       'Post',
    draft:      false,
    created_at: document.date,
    updated_at: document.date
  }
}

File.open('out.json', 'w') do |f|
  f.write JSON.dump(array)
end