Snowballs damaging slimes and magma cubes

To give snow golems (and snowballs in general) more usefulness, snowballs should be able to damage slimes and magma cubes. Basically just add slimes and magma cubes to the list of things that can be damaged by snowballs. It's looking a little sad right now.

This would be useful in farms and swamps. If the bug MC-100375* was fixed, snow golems would be even more useful, as you could take them on a boat in a swamp and they could kill slimes while you're fishing.

*[MC-100375] Snow Golems inside boats doesn't shoot at mobs