Output source maps

This sets the necessary Recast options so that source maps are outputted. This also links the source maps to the generated file via the `//# sourceMappingURL=` comment that's appended.

*Note:* This currently doesn't compose the generated source map with an existing one. Doing so might require adding some options to the `Container`/`Writer` class(es).
I dove in deep on source maps reading posts from @thlorenz (http://thlorenz.com/blog/browserify-sourcemaps) and it seems like people keep the paths between the `.js` and `.map` files to just the file names and assume they are always siblings.