support GTFS-Flex

`gtfs-utils` should support [GTFS-Flex v2](https://github.com/MobilityData/gtfs-flex/blob/e1832cfea5ddb9df29bd2fc50e80b0a4987695c1/spec/reference.md).

- [ ] support `locations.geojson` & `location_groups.txt`
	- [ ] `lib/read-stop-times`
	- [ ] `compute-stopovers`
	- [ ] `compute-connections`
	- [ ] `compute-sorted-connectoins`
	- [ ] `compute-schedules`