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.
[The first line of the documentation string should stand on its own](https://www.gnu.org/software/emacs/manual/html_node/elisp/Function-Documentation.html#Function-Documentation).

This is more important for functions, but as you can see in our codebase even the defstruct descriptions are following this convention (`js2-break-node`, `js2-continue-node`, etc).