Add support for bracketed html attributes

In response to machty/emblem.js#69

This is entirely a first pass, and I'm certainly a bit green on best practices with pegjs.  That said, I've run a number of tests, and within brackets, this should be a fairly flexible solution.  From what I can tell, the nature of the language and the terminators used make it somewhat difficult to add attributes without some sort of markup.  The bracketed markup aligns with slim conventions, (which @machty mentioned was a close relative):

http://rdoc.info/gems/slim#Attributes_wrapper

Everything's been tested, with a new test for this convention. I'm not sure why the build tools wrapped the method in a self executing function, I pretty much used existing build tools without modification.

Again totally open to feedback and I'll be happy to refactor accordingly.