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

Hi, I added ability to apply transformation function to entered char on keypress event.

this can be used like that:
```javascript

$("#po").mask("PO: aaa-999-***", 
    {transform: function(s){
        return s.toUpperCase()
    }
});
```
or
```javascript

$("#po").mask("PO: aaa-999-***", 
    {transform: function(s){
        if (s == 'm') return 'n';
        else return s;
    }
});
```
or something like that