Add a new transform strategy to fix shapes which are used for both directions of travel

Some GTFS feeds use the same shape for trips traveling in both directions along the same path; this appears to violate the GTFS specification, and has been observed to cause problems with OneBusAway, but nonetheless there are feeds in the wild (such as for Virginia Railway Express) which do this.

This pull request introduces a new transform strategy which creates a reversed clone of a shape and applies that new shape to any trips which use the selected shape in the 'wrong' direction.