Better flexibility for 'type' in `WP_Comment_Query`.

* Add support for an array of values in 'type'.
* Introduce `type__in` parameter. This duplicates 'type' but is added for better consistency with other query classes.
* Introduce `type__not_in`.

Among other things, these changes will make it easier for plugin authors to
manage the appearance of custom comment types in various WP interfaces.

Props dancameron, mordauk.
See #12668.

git-svn-id: https://develop.svn.wordpress.org/trunk@30096 602fd350-edb4-49c9-b593-d223f7449a82