Add whitelist option for list-filter to prevent arbitrary tag generation

Some of my mail contains weird list-id headers (e.g. airline mails), tricking afew into creating weird tags. To prevent this from happening i added a 'whitelist' option to the 'ListMailsFilter'. If empty (default), afew behaves just the same. With a non empty whitelist any list mail still gets the 'lists' tag, but only those mails with a matching list-id get the extra 'list-id' tag.