Add support for more CSS/JS minifers

[rondevera]

To keep the AssetHat gem lightweight, this should not just add gem dependencies for various other minifiers -- this would make installation very slow.

As we chatted about, the setup process in a host app should instead be:

1. Update Gemfile to include your minifier of choice.
2. Update assets.yml to use the AssetHat engine for that minifier.

In this way, an "AssetHat engine" is essentially an adapter -- it provides a universal interface for using any minifier. AssetHat would contain a bunch of engines out of the box, and to use one, just add the corresponding minifier to your Gemfile.

Out of the box, AssetHat should still support weak and cssmin for CSS, and weak and jsmin for JS. These are lightweight, pure Ruby, and have few/no other dependencies.