Refactor and test for 100% coverage

Also fixes a couple of things:

* Add test for potential prototype injection
* Fix duplicate definitions to prefer the first to match CommonMark
* Fix to set empty position information, instead of a fake one
* Fix to set empty `alt=""` on images (resources and references) w/o alt text
* Remove defaultProps if handled in code
* Refactor to let unified handle given plugins
* Refactor to let clarify unified plugins vs. custom transforms
* Refactor to significantly improve performance when (dis)allowing nodes,
  and probably also fix a lot of bugs with it (the tags not being stripped
  in some of the issues)

Closes GH-487.