allow customising the DB schema

[derhuerst]

This use case has been brought up in derhuerst/berlin-gtfs-rt-server#9.

berlin-gtfs-rt-server uses hafas-gtfs-rt-feed underneath, which in turn uses gtfs-via-postgres & match-gtfs-rt-to-gtfs. The former already supports using a schema other than public, but match-gtfs-rt-to-gtfs doesn't support it yet.

Two places would have to be adapted:

• The build-index command needs a new option (e.g. --db-schema).
• The actual matching logic (index.js) needs a new option (e.g. dbSchema).

The name of the schema should be added to the DB queries in a secure manner, e.g. by using pg-format.

[derhuerst]

cc @dancesWithCycles