export sub-schedule of select routes

[fpurcell]

gtfsdb firstly will need to write .gtfs files out (not yet implemented) secondly, there will be a process to filter the export based on route id any other filters? dates? region?

extra credit:

• be able to merge multiple agencies (that reside in different schemas) in the export
• that would mean doing something with id's (hyphen ... or one id ... or ???)