add an instance config trait
add read only test
phpcs fixes
phpcs fixes
they should _not_ declare a config property
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
phpcs fixes

and add a test for when deleting should fail
Merge pull request #3116 from AD7six/3.0-config-core

add an instance config trait