Add maximum waiting time to transfers.txt

I was searching in documentations and proposals and did not find any regular way how to express this.

In our system in Czechia we have a lot of cases of trip-to-trip transfers, where the second vehicle waits for the first one, but only for a specified maximum amount of time. For instance a bus waits for a train, but at most 20 minutes. If the train is delayed more than 20 minutes, the bus leaves and the passengers need to wait for the next bus. This value is specified per-transfer, so the best option for us would be to extend _transfers.txt_ by adding a new column `max_waiting_time` which would specify this value in seconds or it could be left empty if the vehicle waits _forever_ or the waiting time could not be specified (this way backward compatibility would be ensured).

The usage is desired in following way

* Applications can inform passengers that the vehicle waits at most X minutes
* More sofisticated planners that incorporate realtime delays could calculate with this value to determine that the transfer could be missed, even though the vehicle is expected to wait

Are there any other producers/consumers using trip-to-trip transfers that would find this useful?