Add remote GTFS repositories

[Kanahiro]

Since v3.0.0, GTFS-GO support remote GTFS repositories.
Users can toggle mode, preset or remote repositories.
Now only [Japan]GTFS data repository can be used as remote.
It is good for other country to be added.

As I'm Japanese I'm not familiar with foreign country repositories.
Please tell us GTFS repository you wish to add here. :)

[takohei]

MobilityData provides the Mobility Database.
https://mobilitydatabase.org/

We can get GTFS catalog information via API or CSV.