Add more GTFS editors

[barbeau]

I just ran across several in this post:
https://gis.stackexchange.com/a/124120/12198

I'm not sure we have all of them on the list. We need to review and add if they are missing, and somehow distinguish open-source from paid proprietary services.

[j05u3]

I agree, also don't forget about datatools.

[carlfredl]

Help wanted: anyone could review this list to see which are still current and draft a PR to add those to the list.