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. 
Let's leave it as it is for now. MCS doesn't really check world names for validity, so if someone tries to name their world `/world:|>new<\` MCS will happily try to do so and probably fail horribly.