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.

bin/notify_file_change

@@ -19,7 +19,7 @@ fi
 # 3. Add the following to your .vimrc
 #
 # function s:notify_file_change()
-#   let root = RailsRoot()
+#   let root = rails#app().path()
 #   let notify = root . "/bin/notify_file_change"
 #   if executable(notify)
 #     if executable('socat')