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. 
So I thought about this some more, I could just drop the `weather WeatherType WorldName` parameter combination for the time being. Then to change a world's, other then the default, weather one would use the full length `weather WeatherType DurationInSecs WorldName` command with Duration set to 0 (the zero will get ignored basically saying just use the default duration). Thoughts?