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.

lib/autospec/rspec_runner.rb

@@ -19,6 +19,10 @@ module Autospec
 
     watch(%r{^spec/fabricators/.+_fabricator\.rb$})     { "spec" }
 
+    watch(%r{^app/assets/javascripts/pretty-text/.*\.js\.es6$}) { "spec/components/pretty_text_spec.rb"}
+
+    watch(%r{^plugins/.*/spec/.*\.rb})
+
     RELOADERS = Set.new
     def self.reload(pattern); RELOADERS << pattern; end
     def reloaders; RELOADERS; end