Refactor more internals to be remark plugins

This is a first step in making a switch to rehype in internals possible.

* Remove `astPlugins`
* Add temporary `htmlParser` prop to pass the htmlparser2 based HTML parser
  (plan: to use `rehype-raw`)
* Fix a couple visitors

Closes GH-547.

Reviewed-by: Christian Murphy <christian.murphy.42@gmail.com>