[RFR] Introducting the valid() modifier

[fzaninotto]

<?php

// valid() only accepts valid values according to the passed validator functions
$values = array();
$evenValidator = function($digit) {
    return $digit % 2 === 0;
};
for ($i=0; $i < 10; $i++) {
    $values []= $faker->valid($evenValidator)->randomDigit;
}
print_r($values); // [0, 4, 8, 4, 2, 6, 0, 8, 8, 6]

// just like unique(), valid() throws an overflow exception when it can't generate a valid value
$values = array();
try {
  $faker->valid($evenValidator)->randomElement(1, 3, 5, 7, 9);
} catch (\OverflowException $e) {
  echo "Can't pick an even number in that set!";
}

Refs #567

[EmanueleMinotto]

missing phpdoc?