Split the code doing highlighting from the rest.

[nickolay]

Currently highlights.coffee mixes distinct functionality:

• finding, loading, and selecting grammars to use via first-mate module. This has many dependencies and is a large part of the whole module.
• returning the HTML for highlighting, given tokens from first-mate's grammar. It is self-contained and pretty straightforward, when you know what it does.

I suggest splitting the two for the sake of those assessing the library.

My story: I was looking for a library to perform syntax highlighting based on AST. I did not find it yet, but found this library.
I got confused by the "Loading Grammars From Modules" section of the README, as it seemed to suggest there was a way to write a JS module to perform the syntax highlighting, but provided no information on the API of such module, so I went looking into the source code to find what kind of input the highlighting code expects from the tokenizer.

(Turns out you only support TextMate-style language definitions, which are handled by first-mate, but the bit of code I'm splitting into a separate file would still be reusable in my case.)

[bcoe]

@nickolay mind rebasing with master, also I'd love to know @kevinsawicki's thoughts on this refactor.

[nickolay]

@bcoe Thanks for getting back to me! Sure, if you decide this split-up is something you want, I can prepare an updated PR.

[bcoe]

@nickolay if you're still interested in landing this refactor, please go ahead and update this pull request and I'll work on getting it landed.