Externalize the {% gist %} tag as the jekyll-gist gem.

jekyll.gemspec

@@ -39,6 +39,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency('redcarpet', "~> 3.1")
   s.add_runtime_dependency('toml', '~> 0.1.0')
   s.add_runtime_dependency('jekyll-paginate', '~> 1.0')
+  s.add_runtime_dependency('jekyll-gist', '~> 1.0')
   s.add_runtime_dependency('jekyll-coffeescript', '~> 1.0')
   s.add_runtime_dependency('jekyll-sass-converter', '~> 1.0')
 

lib/jekyll.rb

@@ -75,6 +75,7 @@ require_all 'jekyll/tags'
 require 'jekyll-coffeescript'
 require 'jekyll-sass-converter'
 require 'jekyll-paginate'
+require 'jekyll-gist'
 
 SafeYAML::OPTIONS[:suppress_warnings] = true
 

lib/jekyll/tags/gist.rb

@@ -1,47 +0,0 @@
-# Gist Liquid Tag
-#
-# Example:
-#    {% gist username/1234567 %}
-#    {% gist username/1234567 file.rb %}
-
-module Jekyll
-  class GistTag < Liquid::Tag
-
-    def render(context)
-      if tag_contents = determine_arguments(@markup.strip)
-        gist_id, filename = tag_contents[0], tag_contents[1]
-        gist_script_tag(gist_id, filename)
-      else
-        raise ArgumentError.new <<-eos
-Syntax error in tag 'gist' while parsing the following markup:
-
-  #{@markup}
-
-Valid syntax:
-  for all gists: {% gist user/1234567 %}
-eos
-      end
-    end
-
-    private
-
-    def determine_arguments(input)
-      matched = if input.include?("/")
-        input.match(/\A([a-zA-Z0-9\/\-_]+) ?(\S*)\Z/)
-      else
-        input.match(/\A(\d+) ?(\S*)\Z/)
-      end
-      [matched[1].strip, matched[2].strip] if matched && matched.length >= 3
-    end
-
-    def gist_script_tag(gist_id, filename = nil)
-      if filename.empty?
-        "<script src=\"https://gist.github.com/#{gist_id}.js\"> </script>"
-      else
-        "<script src=\"https://gist.github.com/#{gist_id}.js?file=#{filename}\"> </script>"
-      end
-    end
-  end
-end
-
-Liquid::Template.register_tag('gist', Jekyll::GistTag)