Automatically add ugoira tag to ugoira posts.

Note that this only adds the ugoira tag to .zip posts. Unlike the other filetype tags, it does not remove the ugoira tag from posts converted from ugoira, but that aren't true ugoira.

IMO ugoira conversions shouldn't be tagged as ugoira now that we're going to have true ugoira support, but I'll put that to the forum to decide.