Fixed a few bugs, made a cleaner error, and added a small feature to help for checking to see if a
Processor instance knows about a file. It's small feature release day!
- better error if resolver returns bad file (513c2a9)
- check for files via processor.has() (b5a539b), closes #533
- remove random async from processor.string (d5ca5c0)
- rollup outputs non .css files in the correct order (2b8fec6)
- add processor.has to check for known files (df5ac6f)