Use a textarea for the commandblock to accept multiple commands

Old commandblocks aren't converted.  (If wanted I could probably add that, but there would need to be a way to reset the formspec, perhaps by punching the commandblock)
This depends on #137 and includes it.
I also set the default command to `tell @nearest Commandblock unconfigured`, which doesn't require any special privileges and won't do anything unwanted if accidentally activated.