Support for formulas

[sswatson]

This module doesn't provide a shortcut for math formulas, and it also doesn't work properly for any of the other shortcuts when a formula appears earlier in the line. This is because formulas show up in a weird way in the textContent attribute of the domNode. Using the Quill objects's getText method works better.

For those who might be interested, I fixed both of these issues in my fork. I can make a PR if there's interest.