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).
Do you think we should use `relativePath` for this instead?