Add hook for integrating with react-quill

[c-w]

This change adds a new top-level function window.mathquill4quill which acts as a factory method for the enableMathQuillFormulaAuthoring call. Decoupling the enableMathQuillFormulaAuthoring call from the Quill instance enables integrating the mathquill4quill library with other projects that are built on-top of Quill such as react-quill mentioned in #9.

[c-w]

@boomanaiden154 given that you've contributed a lot to this project recently, would you mind taking a look at this pull request and let me know your thoughts? Thanks in advance!

[boomanaiden154]

It looks good. The preservation of the functionality outside react-quill would be what I am most concerned about, and this is preserved with a minor breaking change upon update to the latest version(just need to slightly refactor the code enabling mathquill4quill). I don't have much experience with react, but it looks good.

[c-w]

Thanks for taking a look @boomanaiden154. Note that the change is fully backwards compatible and I've verified that the previous method of initialization via quill.enableMathQuillFormulaAuthoring(options) continues to work. The only change is that now we expose a top-level function on window but this shouldn't impact existing applications (unless mathquill4quill is already defined as a global which seems unlikely).

[boomanaiden154]

@c-w guess I didn't see the part at the end that exposed the function with quill.prototype. Everything looks good.