Negative look behind or not()

[apburton84]

the implementation of:

$flux->not('something');

would be of great use to myself

(?!something)

An implementation of:

public function not($string)
{
    return $this->raw(sprintf('(?!%s)', $string));
}

[selvinortiz]

@apburton84 Might be a nice addition. Do you want to submit a pull request?