Add configuration for parent_labeling.

[mlsad3]

@ranmocy and @aaronj1335

As per #72, This adds a config for parent_labeling. By default it is True.

This PR should also include modifying the Sheet's Config page to have the following:

parent_labeling	TRUE

Hover-over Note: Add parent labels to emails. For example if add_labels is 'abc/def', then not only would 'abc/def' be added, but 'abc' (the parent label) would also be added.

Also config errors now throw an exception so they are noticed by users (Alert box is used by Apps Script).