Add option to preprocessTags before creatingTag

Write now, there's no way to change the value of a tag before it gets created. 
For example, you could use this to capitalize every tag before creating it. 

```javascript
$("#tag-it").tagit("preprocessTag", function(val) {
  if (!val) { return ''; }
  return val[0].toUpperCase() + val.slice(1, val.length);
});
// foo -> Foo
```