Add character-set-like matching to numerify

* Add support for specifying sets of digits to be randomly selected from when
generating a string with numerify:
```php
    // Returns a random selection from array( 1, 2, 3 )
    \Faker\Providers\Base::numerify('[123]');

    // A valid UK telephone number e.g. 01422 417355
    \Faker\Providers\Base::numerify('0[127]### ######');
```
* Add phpunit to composer.json