Is service available on x day?

Once we have GTFS data, it might be nice to have a function that tells you if a service is available on a particular day. Currently, you will have to check `calendar` to see if it is normally open, and then check `calendar_dates` to make sure there isn't an exception.

See:

- https://developers.google.com/transit/gtfs/reference/calendar-file
- https://developers.google.com/transit/gtfs/reference/calendar_dates-file

Start by inspecting how this JavaScript library represents GTFS data and whether calendar or calendar_dates handling already exists. Use the linked GTFS calendar references to understand regular schedules and exceptions; done means exposing a function that correctly reports service availability for a particular day.

Written by the indexing model from the issue text.