Expose PhoneNumberFormat class

Expose `PhoneNumberFormat` class to allow call to `phoneUtil.format` method :

```javascript
var libphonenumber = require('libphonenumber');

var PhoneNumberFormat = libphonenumber.PhoneNumberFormat;
var phoneUtil = libphonenumber.phoneUtil;

var phoneNumber = phoneUtil.parse('+33163765112', 'FR');

// 01 63 76 51 12
phoneNumber = phoneUtil.format(phoneNumber, PhoneNumberFormat.NATIONAL);
```