matching improvments

[stefanpenner]

Our current matching behavior has grown fairly organically, but it is likely time to do some cleanup and refactoring.

Current matching quirks:

• include/exclude don't work with files
• include \w only string inputs takes a fast-path when walking the dir tree
  • excludes are applied after (but could be used to improve dir walk pruning)
  • when the fast-path is taken, we still duplicate matching effort here

Goals:

• [ ] allow files to be a function https://github.com/broccolijs/broccoli-funnel/issues/25
• [ ] throw if files + includes|exclude are provided, until they work together
• [ ] when files is included, also filter then with include/excludes
• [ ] make include/exclude + RegExp all take the fast path (this likely requires improvements to my walk-sync fork)

note: the fast-path, mentioned above, is actually to prune dir while walking rather then after walking – more details: https://github.com/joliss/node-walk-sync/pull/5