cli: add --remove-all option to "notmuch tag"

Add --remove-all option to "notmuch tag" to remove all tags from the
messages matching query before applying the tag changes. This allows
removal of all tags and unconditional setting of the tags of a
message:

$ notmuch tag --remove-all id:foo@example.com
$ notmuch tag --remove-all +foo +bar id:foo@example.com

without having to resort to the complicated (and still quoting
broken):

$ notmuch tag $(notmuch search --output=tags '*' | sed 's/^/-/') \
  id:foo@example.com
$ notmuch tag $(notmuch search --output=tags '*' | sed 's/^/-/') \
  +foo +bar id:foo@example.com