Add transit plugin for the Denver RTD (Regional Transportation District)

It seems the API documentation is available here: https://www.rtd-denver.com/open-records/open-spatial-information

"We are committed to supporting the development community in whatever way we can. We supply static schedule data in the General Transit Feed Specification (GTFS), real time vehicle locations and arrival predictions in the GTFS-RT format, as well as GIS files in ESRI shapefile formats. All of this data is available for you to use for developing your app."