Option to set clear value when clicking "x"

This allows you to override the default of clearing to a value of ```""```, for example you could set ```clearValue: "0"``` to make the x created by ```allowClear``` reset to ```option[value="0"]```.

I added this because I have legacy code where sometimes the default value is ```0```, thought it might be useful to give this option to others, it will work with any string.

Default is set to ```""``` like before.