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).
Fix formmating to two space indentation
Update test runner for source maps
Update recast to v0.6.6

This fixes an issue with source maps for modules whose code would be a noop, like a chained import/export:

```
import { value } from './first';
export { value };
```
Merge pull request #137 from ericf/source-maps

Output source maps