Weather control

[xoft]

It'd be nice to have a plugin providing a more thorough weather control. Currently, the core can change weather, and that's it. However, some people might want their weather never to change, or change on command. Thus, I'm proposing a new set of weather commands, either in a separate plugin, or as part of the Core:

- /weather <weather> - sets current weather to the one specified, regular weather-cycling is enabled
- /weather keep <weather> - sets current weather to the one specified, weather-cycling disabled (the weather stays forever)
- /weather change - changes the current weather to a different one, doesn't affect weather-cycling
- /weather interval <weather> <interval> - sets the interval used for regular weather-cycling for the specified weather.
- /weather intervals <sunny-int> <rainy-int> <thunder-int> - sets the intervals used for regular weather-cycling.