Add abbreviate_artistsort plugin.

Abbreviate Artist-Sort and Album-Artist-Sort Tags.
e.g. "Vivaldi, Antonio" becomes "Vivaldi, A."
This is particularly useful for classical albums that can have a long list of artists.
%artistsort% is abbreviated into %_artistsort_abbrev% and
%albumartistsort% is abbreviated into %_albumartistsort_abbrev%.