Add Sofia GTFS data

[isabelle-dr]

Following up on this LinkedIn post announcing that Sofia has published its public transport data openly in GTFS and GTFS RT.

This issue is for:

• validating the official source for the static data (the one we already have, or a new one)
• add the realtime data (url TBD, I couldn't fint it from a quick search)

[jbruechert]

See public-transport/transitous@54f30bc for the commit adding the new feed in Transitous.

It also has the realtime urls.