Add a Markup Editor toolbar button

Hope this falls in-line with the types of features you're looking to implement. Editing raw HTML has been a invaluable feature for editors I've used in the past and is a requirement for me to use quill.

There are a couple problems I'm currently blocked on:
- I was unable to figure out how to get a icon for the toolbar
- I didn't see any obvious unit tests for custom `Tooltip` objects.
Let me know if you have any recommendations on how to approach these two issues.

Also, I think it would be helpful to include something along the lines of [behave.js](https://github.com/jakiestfu/Behave.js). I didn't want to introduce that dependency, so the markup is currently unformatted.