Add -S and --search to specify a named saved search

Accept the name of a saved search. Works logically in all forms:

* When only a search is specified, use the search and group to which it is attached
* When a search and a group are both provided, use the user-supplied group (so that one search can be applied to a different group)
* When a search and a group are both provided, prefer searches within that group (to disambiguate between searches of the same name in different groups)