Add support for shimming modules

[bouk]

• Add support for passing in babel transform options in the processor config
• Add support for shimming 'process' and 'Buffer' global

@lemonmade @alanly

This will make it possible to use enzyme with sprockets-commoner, so you can blacklist any modules that we don't need (namely those listed at https://github.com/airbnb/enzyme/blob/master/docs/guides/browserify.md).