Add `max_rec` configuration options.

Allow users to apply a maximum recommendation to partial albums and albums
with differing track lengths/numbers.
Merge pull request #98 from mrmachine/low-rec-triggers

Add `low_rec_triggers` setting.
style tweaks & doc expansion (#98)
max_rec (#98): more lenient defaults

These defaults more closely reflect the current behavior of beets, which I
think is desirable.
max_rec (#98): fix bug w/ multiple track triggers

If both "tracklength" and "tracknumber" triggers occur for a certain album,
then the *lowest* recommendation max should be used. (With the `break`s in
there, it would just be the first trigger found.)
path_sep_replace config option

I also took this opportunity to move and rename util.santize_for_path to
library.format_for_path, which was long overdue.
configurable max filename length
look up max filename length automatically
docs/changelog: filename length detection (GC-461)
original_{year,month,day,date} fields (GC-441)
original_date config option (GC-441)