Add platformcodes

This add supports for platform_code in GTFS's stops.txt to Navitia, which is a almost official part of GTFS specification [1]. It's used in the GTFS feeds of 9292opendata.org and gtfs.ovapi.nl

[1] https://groups.google.com/forum/#!searchin/gtfs-changes/platform_code/gtfs-changes/UtoaTOKT1TE/C6ZU8K_g6fUJ