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.
Please try to keep the code within the standard 80-column limit.

Here, you can improve the nesting level somewhat by adding a newline after the `cond`, which is the usual convention anyway. And there are two enveloping `cond` forms here.