Simplifies URL rebasing.

Simplifies URL rebasing.

** WARNING ** - this is a truly breaking change.

There is now a single option called `rebaseTo`, which denotes a
path all relative URLs are rebased to.

Absolute URLs are kept intact as it's assumed user knows what
he/she does when using them, however absolute @imports are still
inlined relative to the current working directory.

This commit also re-introduces ability to pass arrays to CleanCSS
`minify` method.

** IMPORTANT ** - it'd probably be wise to give users an option
to use a function as `rebaseTo` so they can restore some of old
functionality if needed.

Why:

* The old system with three options (`relativeTo`, `root`, and
  `target`) was too hard to understand and work with reliably.