Cleanup of the Plugin class

Bug 1328768

- Improving documentation

- Adding default implementations for the *_config_options methods because
that's a better means of documenting them than the function_exists() way

- Fixing the save_config_options() methods in the search plugins, which
should have been static.

Change-Id: Ib2bf1388843824e63cf6486176e97b8f874e1116
Signed-off-by: Yuliya Bozhko <yuliya.bozhko@totaralms.com>
Merge "Cleanup of the Plugin class"