Feature: CommonMark support

This adds the markdown.it engine to Discourse.
https://github.com/markdown-it/markdown-it

As the migration is going to take a while the new engine is default
disabled. To enable it you must change the hidden site setting:
enable_experimental_markdown_it.

This commit is a squash of many other commits, it also includes some
improvements to autospec (ability to run plugins), and a dev dependency
on the og gem for html normalization.

test/javascripts/test_helper.js

@@ -22,6 +22,7 @@
 //= require vendor
 //= require ember-shim
 //= require pretty-text-bundle
+//= require markdown-it-bundle
 //= require application
 //= require plugin
 //= require htmlparser.js