Add Gist code.

Author: parkr

lib/jekyll-gist.rb

@@ -1,7 +1,43 @@
 require "jekyll/gist/version"
 
 module Jekyll
-  module Gist
-    # Your code goes here...
+  class Gist < 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)