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.

app/assets/javascripts/pretty-text/engines/discourse-markdown/code.js.es6

@@ -21,6 +21,8 @@ registerOption((siteSettings, opts) => {
 
 export function setup(helper) {
 
+  if (helper.markdownIt) { return; }
+
   helper.whiteList({
     custom(tag, name, value) {
       if (tag === 'code' && name === 'class') {