Add "disableEmpty" option for automatically disabling empty multiselects

When I'm using this plugin I want to disable all lists that have no options in them. This adds an option for this.
![example of disabled input](https://cloud.githubusercontent.com/assets/2562231/3828403/84a7ed98-1d77-11e4-9040-a44422d09582.png)

Sorry for the line endings thing, looked good in Visual Studio when I merged! Any ideas of how I can make my pull request more clear is appreciated.