Disable MathJax by default.

GitHub.com doesn't support MathJax. The parser is not MathJax aware which causes problems.

--mathjax enables MathJax.

bin/gollum

@@ -65,8 +65,8 @@ opts = OptionParser.new do |opts|
     wiki_options[:live_preview] = false
   end
 
-  opts.on("--no-mathjax", "Disables mathjax.") do
-    options['mathjax'] = false
+  opts.on("--mathjax", "Enables mathjax.") do
+    options['mathjax'] = true
   end
 end