determine read/write from the number of func arguments

rather than whether the value is null. Prevents problems with for
example:

	$inst->config('setthis', 'some value'); // or the default
	...
    $something = $couldBeNull;
	$inst->config('setthis', $something);

which would be pretty confusing.

Also add the possibility to delete a config key - same use case/example
as above