Processor Cache Keys

Implementation for #671.

Processors (engines or transformers) MAY define a method called `cache_key` that returns a static cache key identifying the state of the function. (Method name maybe revised)

* [ ] Cleanup `uri => processor cache key` function
* [ ] Needs test cases

/cc @chancancode @elia 
So we can add the current sass version to the key. But theres no much we can do about function monkey patches and library changes.