Continue supporting functions for src and dest in middleware

I noticed that [stylus supports functions](https://github.com/LearnBoost/stylus/pull/930) for `src` and `dest` options in it's middleware. This is cool and allows me to retain a file structure I prefer over mirroring my public directory. However it seems like a [commit to resolve overlapping paths](https://github.com/LearnBoost/stylus/commit/01f870813b2af0aa8950d661f609c94f5cf23c24#L0R234) broke this functionality by throwing a "cannot call split" error. This will fix this, allowing functions again.

I couldn't find tests for the middleware, but will be happy to write some if you can show me where I should write them. If there are no tests for middleware I'd be happy to get that started too.