GTFS-Fares v2: Improvement of filling with stops of a certain area

It would be convenient to have possibility to bundle-fill the area.

Motivation: in Athens there are `airport` area and `urban` area. The `airport` area can be simply modeled by adding stops to the `stop_areas.txt`. The `urban` area contains all other stops of the city and there's a lot of such. Therefore if a provider adds some stops to the stops.txt in the `urban` zone he could miss to add them to the `stop_areas.txt` and it will fall to an issue.

I’m proposing to add new file `area_networks.txt` as many-to-many relation between `network_id` and `area_id`. This means that all stops of the routes marked by `network_id` will belong to the `area_id`.

Additionally I’m proposing to extend `stop_areas.txt` by `type` (with possible values `include` or `exclude`) which will defaults to `include`.