added ability to apply transformation function to entered char on keypre...

[Sorgor]

Hi, I added ability to apply transformation function to entered char on keypress event.

this can be used like that:

$("#po").mask("PO: aaa-999-***", 
    {transform: function(s){
        return s.toUpperCase()
    }
});

or

$("#po").mask("PO: aaa-999-***", 
    {transform: function(s){
        if (s == 'm') return 'n';
        else return s;
    }
});

or something like that

[digitalBush]

This is a neat idea. To be really useful though, I think we might want to expose a bit more context to the callback function. I'm going to look into something like this for 2.0.