Add option for custom js (like custom css)

* Add a new 'js' flag to indicate you want to embed a file named 'custom.js'
  which should exist at the root of the wiki

bin/gollum

@@ -49,6 +49,10 @@ opts = OptionParser.new do |opts|
     wiki_options[:css] = true
   end
 
+  opts.on("--js", "Inject custom js. Uses custom.js from root repository") do
+    wiki_options[:js] = true
+  end
+
   opts.on("--page-file-dir [PATH]", "Specify the sub directory for all page files (default: repository root).") do |path|
     wiki_options[:page_file_dir] = path
   end