Modularizing the API

[UltCombo]

es20xx is great at handling JS workflows, but I don't want to include the whole front-end tooling (e.g. CSS and HTML preprocessors) in the gulpfile.

I believe we should be able to provide a way for users to require() the build and watch tasks in their gulpfiles, so they can incorporate es20xx into their existing workflows and seamlessly update es20xx without manually editing the gulpfile.

This will not change the current slush generator, but rather expose an API that registers the es20xx gulp tasks. I'll probably split the slush-es20xx API into another package, then users can require the API directly.

Thoughts?