This PR closes #104 with some modifications to how the files were broken apart.

Overview

One function per file felt a little too shallow, so I broke things apart three major modules–customizers, overrides, and utilities–and also moved everything to src/:

src
├── customizers
│  ├── babel.js
│  ├── index.js
│  └── webpack.js
├── index.js
├── overrides.js
└── utilities.js

customizers

These are all the functions that actually modify the config files. They're currently grouped by webpack and babel, which may not be entirely accurate, but I don't want to break this down anymore without more thought and discussion.

Could eventually be split down even more into enablers that actually enable functionality and features and customizers that simply modify existing configuration. Or something. Not sure it really even matters.

overrides

This is just override and overrideDevServer. They felt special enough to house in their own file.

utilities

Currently just getBabelLoader. Should eventually have more functions that enable consumers to build their own plugins.

src/

I went ahead and moved everything over to src/ in preparation for #113.