ES6 module syntax support

Provisional support for es6 module import/export syntax.

Because we use this syntax is a growing majority of our projects, I had to implement it. Obviously this doesn't track a spidermonkey implementation that doesn't yet exist, but for those that want provisional support, it could prove very useful (per the discussion in #139)

I'd love any feedback.
I think this should also start with "Parse". And we usually write the constant name for the required current token instead of a character (`js2-MUL` here?).