Support for custom transforms

[nfour]

The ability to have custom transformers, separate to the default Uglify and Babel.

• [ ] Add transformers by a filename pointer (like handlers) in the config
• [ ] Refactor transformer logic to resolve above
• [ ] Let transformers determine file extension matching via static props
• [ ] Force transforms to manipulate files, not in-memory blobs, for performance
• [x] Uglify should produce an inline source map like Babel