Support for custom transforms

[Sam J]

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
• Uglify should produce an inline source map like Babel