Add parameter highlighting for ES2015’s arrow functions.

[savetheclocktower]

Also: properly highlight constants like null and false when they are default values of parameters.

Before:

After:

I borrowed portions of these patterns from this JavaScript (Babel) language definition for Atom. I do agree with #51, though, that ultimately the best solution would be to port that grammar over to TextMate; some of its scope names would need to be fixed, but it's probably less effort than bringing things over piecemeal.