Allow unicode in rewrite items
Allow unicode in rewrite items

This commit makes rewrite explicitely match items using the .lower() function
instead of using Pythons builtin re.I flag.

This is required as the re.I flag only allows for case-independent pattern
matching with ascii chars. Even worse, the pattern is stored in lowercase when
using re.I, but the value to be matched isn't lowercased.

Example:

[rewrite]
artist ?????? ?????????? ??????????: Sergei Rachmaninoff

google-code-export/beets,google-code-export/beets,google-code-export/beets