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)
require python-musicbrainz-ngs 0.2 or newer

This came up because version 0.1 (which is available in some Debian repos)
apparently lack `set_hostname`.
fetchart: remote_priority config option
Sanitize template evaluation for smartplaylist