Added option to use an optional callback to filter the actual number.

While working on a recent project I needed a way to display the spinner number in european format (with a comma) using the step feature. 

So 5.5 should be 5,5 - 10.5 should 10,5 etc. I modified the code so that I could pass a callback function to filter the output however i wished.

It would be nice if this could be added to the core spinner code as this is a nice feature that does not break any existing functionality.