Add means of using gtfs extensions for NYC MTA and others

[dmf3030]

This might already exist in the library but I haven't found it. The MTA and presumably some other agencies have custom gtfs extensions to expose extra data but I can't find a way to get those custom extension loaded into this library.

The MTA one can be found here

MTA Extension

[tolidano]

https://github.com/Andrew-Dickinson/nyct-gtfs Take a look at this project, which does what you want.