Add more US cities

[mpadge]

But not NY or LA, because they're too huge to just casually recompute at regular intervals. They'll need to wait for consistent $ to support that. So just add other cities of more manageable size for which GTFS data are available. Possibilities:

• Chicago
• Minneapolis / St Paul
• Denver
• Salt Lake City
• Santa Fe
• Phonix
• Las Vegas
• Dallas
• Austin
• Tampa
• Miami
• Boston