use the yml config system instead of _config.php for extensions
Merge pull request #2817 from lerni/patch-1

use the yml config system instead of _config.php for extensions
BUG GridState_Data doesn't hold falsey values

After patch #2522 was taken GridField_Data was no longer able to hold
falsey values. See failing tests.
BUG Fixed handing of false values in GridState_Data
API Added ability to unset values