Expand $transition API a bit

I was doing some work on #911 and it seemed like these changes would have been
useful for it.

It turns out that they aren't really needed, but I thought it wouldn't hurt to
see if anyone thought they were useful/worth keeping.

There are a couple of tests added for the new functionality, which includes:

- Pass array of triggers (EG for adding CSS classes as well as style CSS or
  functions)
- Support variety of CSS class semantic actions (prepend +,- or ^ to add, remove
  or toggle classes -- but add by default)

Anyways, just throwing these up there in case someone thinks they're useful, but if
not, as I've said I don't think they're really needed to fix #911 anyways. Cheers