export sub-schedule of select routes

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 ???)