Objects with reverse gravity

[thejoshwolfe]

First suggested in #24 by @Patashu.

Makes me think of the balloons from Heartlight. I wonder what the rules would be for when you compare the weights of objects; i.e. would a size 4 snake weigh down a size 3 balloon or be lifted by it? Could an normal object and a balloon-like object cancel out their weights and hover? Seems like a lot of cool stuff could be done with that idea.

Speaking of gravity, there's also always the age old trope of reversing/redirecting global gravity. That might be cool too.

[XeroOl]

Zero-gravity blocks would also be cool, and possibly simpler to implement. (Just don't apply gravity to zero-gravity blocks, they stay in place unless pushed like a normal block)