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. 
Of course it can't be guaranteed, but MCS makes best effort. Same as vanilla, I guess. We try to run at 20 tps, but if the hw doesn't allow that, we don't do anything special.
I personally think that everything should be tick-based, not second-based, but if it is to match vanilla, then use a clean 1:20 ratio.