work in progress
Major rewrite of assets loader.

- introduce Transformer trait and view all transformations in that light (concat, minify, etc)
- Source file look up is always done via Play.resource
- Generated files are always placed in the same folder target/.assets-loader-generated
- Add a tag cache so if the tag has already been generated - return that instead.

Public api is identical

Note: Deployment is not working for now.
tidy output