Add an AST and AST-parsing mode

[domenic]

This would be the first step for building an "emdfmt" tool that reformats your Ecmarkdown exactly how I like it.

As a bonus, I suppose you'd want to re-build the HTML outputter as an AST visitor.

[domenic]

This is largely done, via #22, although we can hold off on exposing it until I actually want to build such a tool. But yeah, just use the tokenizer then the parser and call parseDocument and you'll get yourself an AST. Such a tool would use a custom emitter.

[bterlson]

#22 solved everything but world peace!

[bakkot]

This is done. Ecmarkup consumes the AST directly in the process of linting, so it's useful even in the absence of a custom emitter.