allowing strings for boolean options to better support BEHAT_PARAMS

See #27

Implemented in terms of [FILTER_VALIDATE_BOOLEAN][1], so `"1"`, `"0"`, `"true"`, `"false"`, `"on"`, `"off"`, `"yes"` and `"no"` are now acceptable for the boolean options `kernel.debug` and `mink_driver`. Everything else will fail validation.

[1]: http://php.net/manual/en/filter.filters.validate.php