Option to not have leading slash on JSONFilter path-type text unit ids

The JSON filter can use an JSONPath-like path as the ID of the text unit. It is in the form: /name1/name2/name3

There is a need for the same value but without the leading /. For example: would give menu/popup/menuitem instead of /menu/popup/menuitem See https://groups.yahoo.com/neo/groups/okapitools/conversations/messages/5334 for details.

This should be an option (not set by default).