Optionally support GTFS-RT data

[devinmatte]

Right now we use the v3 streaming API, which works well for the MBTA, but, there may be data in GTFS-RT that isn't in the API and, more importantly, other cities only have GTFS-RT

If we can set up gobble to optionally also read from GTFS-RT feeds, we can potentially build datasets for other transit systems