Add tests
Rename custom_concat_usemin test to custom_usemin
Default usemin to null
Refactor normalization to only add to .js targets

Fixes #44
Remove .js concat target restrictioni from
0d8ed0d52b9f648fc2ed7706bcc6f060deb213eb
Abstract out concat code
Update concat_usemin test
Add optional filter to appendToConcatTarget
Remove unecessary target
Add support for usemin option
Remove unnecessary dependencies
Clean up code