Support for custom rate limits

[virtualize]

Motive: #19

[nikolaposa]

@virtualize What do you think about alternative approach, of adding new named constructor that would allow custom values, and keeping built-in constructor as is? For example:

public static function custom(int $operations, int $interval)
{
    return new static($operations, $interval);
}

$rate = Rate::custom(500, 3*3600)

[virtualize]

Like the idea very much and updated the PR. Thanks.