Updates to the `weather` console and in-game commands

As discussed in mc-server/Core#97, updated `weather` console and in-game commands to handle different worlds similar to `time`. Also added `[duration in seconds]` option to match the vanilla command. 
The answer is vanilla, craftbukkit, cauldron, spigot, glowstone, etc. don't support any combinations with world names since they are not multi-world servers (if they did, it be easy since I'd have just emulated their solution to begin with). That said, if you're good with the status quo, I'm leaving it as is.