Obtaining before and after value in hook_civicrm_custom when using GUI

Currently, when editing custom field values from the GUI, hook_civicrm_custom acts as though all custom values in the custom group have been changed regardless of whether or not they have been. This makes it impossible to determine which custom fields have changed, and thus difficult to perform an action using the hook on the change of a specific custom field without having each custom field in individual groups.

This fix will provide both the pre and post custom field values for the entire custom group inside of hook_civicrm_custom, so that the user can compare and act on differences.