Add discard border method to FilterObjects

I've simply copied the method used by IdentifyPrimaryObjects to discard border objects into FilterObjects as a new mode (Rules, Measurements, Border).

It would be useful if one wants to use ReassignObjectNumbers to unify objects which were improperly split by IdentifyPrimaryObjects prior to discarding.

I had implemented this as a separate module, but it makes sense to me to include it as part of FilterObjects, since it is a filter. Though if it is preferable, I can do a pull request for the separate module instead.