Feature request - expose Haul as a library

Thank you guys for your amazing work! The need for Webpack-friendly React Native packager is really strong since it is very flexible, number one bundler at the moment and the fact that React Native has its own incompatible packager raises many problems with code sharing between mobile frontend and web frontend or even backend of the same application.

I would like to open discussion about exposing Haul as a library with API for:
- Making React Native webpack config with Haul entry points, resolvers, loaders, babel addons
- Using Haul as a webpack middleware, in similar fashion as webpack-dev-middleware
will enable developers to deeply integrate Haul with their Webpack projects in a programmatic way. Much like the Webpack itself can be attached in a programmatic way to project via `webpack-dev-middleware` and instantiating Webpack compiler instances directly from Node.js code, without the need for launching command line Node.js process.

This is needed for projects that already use Webpack in a programmatic way via its Javascript API and want to include Haul as well the same way.