Allow lone "not" search operators

As suggested by Keith in FLAG_PURE_NOT allows for expressions like:

  notmuch search NOT tag:inbox

Note that this way a search like:

  notmuch search foobar NOT tag:inbox

should not be written instead:

  notmuch search foobar AND NOT tag:inbox

In my opinion, the latter feels more natural and is somewhat more explicit.
It gives a better clue of what the search is about instead of assuming that
an implicit AND operator is there.