Load several static GTFS feeds (multi-feed phase A: configuration) #1976

Multi-agency regions need one static feed for each agency, and each feed has its own default agency. Java OneBusAway loads a bundle that is built from several feeds. Maglev can load only one static feed.

The spec is the wiki page Multi-Agency-Static-Feeds-Stop-Consolidation. Phase A adds the configuration only:

• A gtfs-static-feeds list in the JSON config. The old gtfs-static-feed key continues to work.
• -dump-config shows every feed.
• A parser for obacloud's bundle-inputs.json manifest.
• A gtfs-bundle-inputs-url option that reads the feed list from a manifest.

Until phase C, the importer loads only the first feed.

PRs: #1812, #1813, #1814, #1815.