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. 
When in doubt, consult vanilla, or more likely in this case, Bukkit or Spigot or whatever is alive nowadays. What combinations do they support?

I'm pretty okay with the status quo, I don't see much use for number-named worlds, and if an admin does that, well tough luck, you brought it on yourself.