add --mathjax [CONFIG] to inject root-repo/[CONFIG] file,

which is similar to the behavior of --css and --js.
CONFIG is "mathjax.config.js" by default.

lib/gollum/views/page.rb

@@ -110,6 +110,10 @@ module Precious
         @mathjax
       end
 
+      def mathjax_config
+        @mathjax_config
+      end
+
       def use_identicon
         @page.wiki.user_icons == 'identicon'
       end