Add options to arbitrarily transform Webpack and Babel configs

[davidtheclark]

We could add options that accept functions, which receive the configs Batfish creates and return a transformed config, which Batfish will then use.

These options would be an escape hatch of sorts, allowing for any arbitrary transformations without the need for Batfish to add new options. They would come with the caveat that they give you the power to break Batfish, so you should only use them if you have taken the time to understand the configs that Batfish provides.

[davidtheclark]

Webpack config transforms are in the integration-changes branch as webpackConfigClientTransform and webpackConfigStaticTransform.