the bucket plugin transforms a pattern like 'A - D' into something that
matches all letters starting with A to D (upper and lower case). This
change allows to put more artists into that bucket. E.g. artists
starting with ä or Ä and 0 to 9 and … (three dots)

Example config for overriding a bucket pattern:

  bucket:
     bucket_alpha: ['A - D', 'E - L', 'M - R', 'S - Z']
     bucket_alpha_regex:
       'A - D': ^[0-9a-dA-D…äÄ]
A few artists use three digits, e.g. 112 or 209. They are detected as a
year instead of an artist. Improve the heuristic by only allowing four
digits to be a year.
Merge branch 'sotho-master'
Changelog for #851 (bucket improvements)