Date format validation rule

This allows for validating a PHP [date format](http://php.net/manual/en/datetime.createfromformat.php#refsect1-datetime.createfromformat-parameters).

Rules would look like this:

    'start_time' => 'date_format:"H:i"',

Note the quotes wrapping the value so that the colon does not count as parameter separator.

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Everything documented and properly unit-tested.