Update options without reinitialization

[suluke]

This is a copy of longbill#208
This would really be nice to have. I also imagine this could be implemented much easier in our fork than in the original repository by longbill.

[suluke]

I currently think of a modifyOption(key|path, value) method which triggers an datepicker-option-change event. This way, we can progressively enhance all plugins and core-functions to listen for these events and react accordingly. The overall effort to fully implement this is quite high, though, so we should prioritize which options would benefit the most from this.