Make Parser slightly more noob-friendly

A comment Carsten made in the Kea training class, is that when people cut and paste config snippets, they frequently forget that the last item in a list of options cannot be followed with a comma. This results in a hard failure by the parser.

The suggestion is, could the parser instead emit a warning along the lines of "we see you have a trailing comma . Did you forget something?" and basically ignore that last comma.