Optionally use faster string distance functions

[sampsyo]

I set up a benchmark for album matching to investigate #609 and found that (after fixing some obvious problems) well over half of the matching time can be spent on edit distance calculation. There are some reasonable-looking C implementations of Levenshtein available:

• editdistance
• python-Levenshtein
• jellyfish

We should consider letting beets use these optionally if they're installed (and fall back to our simple Python implementation if they're not).

[ghost]

this could be closed now right? since jellyfish is now required.

[duailibe]

@jrobeson indeed. Thanks!