Support for special characters

Hi, I'm trying to use the `formatCharacters` feature to transform special characters. Specifically japanese numbers (e.g., `１`, `２`, etc.) to the right alpha numbers.

I tried configuring it through `formatCharacters` but the `transform` function seems to never trigger. I did some debugging and tracked the problem to this line https://github.com/insin/react-maskedinput/blob/master/src/index.js#L72 which basically will have `e.target.value` set to `1２/__` but `this.mask.getValue()` overwrites it to `1_/__`.

Please let me know if I'm missing something or if there's another way to do this.

Thanks