Implement buffing tags directly

[jleclanche]

Similarly to direct tag refresh, we should be able to buff card tags directly, eg. Buff(TARGET, {GameTag.HEALTH: SET(1)}).

This would remove the need for various custom buff cards. Especially because of this:

Not implemented: Set Health to 1 (XXX_100)
Not implemented: Set health to full (XXX_101)
Not implemented: Add 1 to Health. (XXX_102)
Not implemented: Add 2 to Health (XXX_103)
Not implemented: Add 4 to Health. (XXX_104)
Not implemented: Add 8 to Health. (XXX_105)
Not implemented: Set Health to 1 (XXX_107)
Not implemented: Set all minions to 1 health (XXX_108)