Gtfs-lite

What's Changed
* Fixed 34 with a frequency matrix rewrite by wklumpen in https://github.com/wklumpen/gtfs-lite/pull/35
* Update gtfs.py to fix reading of attributions.txt by carlhiggs in https://github.com/wklumpen/gtfs-lite/pull/37
* Update gtfs.py to strip df header whitespace by carlhiggs in https://github.com/wklumpen/gtfs-lite/pull/39
* Parse frequencies.txt end and start times as time deltas by carlhiggs in https://github.com/wklumpen/gtfs-lite/pull/41
* Change method from append to extend to concatenate lists by olalid in https://github.com/wklumpen/gtfs-lite/pull/43
* 'exact_times' in frequencies.txt is optional, so should be Int64 not int to accomodate possibility of NaNs by carlhiggs in https://github.com/wklumpen/gtfs-lite/pull/47
* Handle weird additional files. Closes 50 by wklumpen in https://github.com/wklumpen/gtfs-lite/pull/51
* Enable writing to a zipfile. Closes 32 by wklumpen in https://github.com/wklumpen/gtfs-lite/pull/52
* Added feature to remove a route. Closes 49 by wklumpen in https://github.com/wklumpen/gtfs-lite/pull/55
* Updated unique trips at stops function by wklumpen in https://github.com/wklumpen/gtfs-lite/pull/58
* Updated service hours to include frequencies by wklumpen in https://github.com/wklumpen/gtfs-lite/pull/60
* Date coverage and trips update for 48 by wklumpen in https://github.com/wklumpen/gtfs-lite/pull/63
* Added kwargs to file loading for 61 by wklumpen in https://github.com/wklumpen/gtfs-lite/pull/65

New Contributors
* carlhiggs made their first contribution in https://github.com/wklumpen/gtfs-lite/pull/37
* olalid made their first contribution in https://github.com/wklumpen/gtfs-lite/pull/43

**Full Changelog**: https://github.com/wklumpen/gtfs-lite/compare/v0.2.0...v0.2.1

Updated documentation, additional features such as headway matrices, and additional time slice possibilities.

Data loading now accommodates files formatted with leading spaces after commas.

Update to accommodate GTFS zipfiles that are nested (i.e. the folder containing the files was zipped). The load function now searches the entire subdirectory for files and tracks their location.

Bugfix for v0.1.4 to update the `trips_at_stops` function.

Updated with some bugfixes related to date strings, and the addition of a method to find unique trips that visit a list of stops.