Summary:
Allows for optional generation of source maps while building plugins.
Caveat: This will leave a broken `//# sourceMappingURL` comment at the bottom. If you set it to `null`, as the documentation suggests, you will instead get an inlined source map in addition to the written one.
Reviewed By: nikoant
Differential Revision: D29265385
fbshipit-source-id: 1e21e49d2516ecc5909b086e7797736b298b86ab