Allow CiviMail to send to a group filtered by a search (custom or standard)

Users should be able to select a search (custom or standard), a group that the search operates on (and also to process subscribes/unsubscribes from) and arguments to send to search

The arguments will restrict the number of contacts returned by the group. I suspect we can use this feature to do A/B testing of mailing campaigns etc (by creating a custom search to get a random % of contacts)