More helpful debugging when changing to a relative directory.
If we know that the display won't change without input, don't unnecessarily recompute or recopy stuff.  Indirectly fixes #17.
Nicer debug output for directories.
Separate ranking and filtering regex logic.

Basically, we use the faster regex (match as fast as we can) to filter
and the better regex (optimize for clusters of strings) for ranking.
True, we have to run both on all matches, but in any meaningful example,
this'll be preferable to running the slow regex on everything.

Obviates #16 for now.
Releasing version 0.4.2