Plugins: add GitHubify plugin

Originally from Bitcoin.org by the same author as this commit.

Author: harding

_plugins/githubify.rb

@@ -0,0 +1,73 @@
+# This file is licensed under the MIT License (MIT) available on
+# http://opensource.org/licenses/MIT.
+
+## githubify.rb automatically adds links to pull requests, issues, and
+## commits using pattern matches
+
+## Example (URL is the repository to link to):
+## {% githubify https://github.com/bitcoin/bitcoin %}
+## ...content...
+## {% endgithubify %}
+
+module Jekyll
+
+require 'yaml'
+
+  class GitHubifyBlock < Liquid::Block
+
+    def initialize(tag_name, text, tokens)
+      super
+      @repository_url = text.strip()
+    end
+
+    def render(context)
+      output = super
+
+      ## Convert #1234 into URL for the pull request
+      ## If #1234 links to an issue, GitHub automatically redirects
+      #
+      ## Require at least two digits to reduce false positive matches
+      output.gsub!(/#([0-9][0-9][0-9]*)/){ |s|
+        '<a href="' + @repository_url + '/pull/' + $1 + '">' + s + '</a>'
+      }
+
+      ## Convert `123abcd` into URL for the commit
+      #
+      ## Only operate on 7 to 10 chars to reduce false positive matches
+      output.gsub!(/`([0-9abcdef]{7,10})`/){ |s|
+        '<a href="' + @repository_url + '/commit/' + $1 + '">' + s + '</a>'
+      }
+
+      output
+    end
+  end
+end
+
+
+## Code to run if plugin is disabled
+module Jekyll
+
+require 'yaml'
+
+  class GitHubifyBlockDisabled < Liquid::Block
+
+    def initialize(tag_name, text, tokens)
+      super
+    end
+
+    def render(context)
+      output = super
+
+      output
+    end
+  end
+end
+
+#Do nothing if plugin is disabled
+plugin_name = "githubify"
+if !ENV['ENABLED_PLUGINS'].nil? and ENV['ENABLED_PLUGINS'].index(plugin_name).nil?
+  print plugin_name + ' disabled' + "\n"
+  Liquid::Template.register_tag(plugin_name, Jekyll::GitHubifyBlockDisabled)
+else
+  Liquid::Template.register_tag(plugin_name, Jekyll::GitHubifyBlock)
+end