Add optional rename_regex configuration option to allow stripping
patterns from pathnames when renaming them. This is useful when other
programs depend on specific substrings being unique like mbsync which
erroneously assumes UIDs are unique instead of UID+UIDVALIDITY+mailbox
name like the IMAP standard specifies.
Closes#463
Signed-off-by: Manos Pitsidianakis <manos@pitsidianak.is>