Add API endpoint to see available sources to choose from.

[jdeweese1]

While working on the Python wrapper for this API, we were updating the code to be able to decide which service to pull data from (jhu,csbs, etc) we decided it would be advantageous if we could query an url endpoint to get a list of valid sources.

see COVID19Py for more details

(on mobile so I apologize for the formatting)

[ExpDev07]

#149

[ExpDev07]

Available sources can now be found at the endpoint: https://coronavirus-tracker-api.herokuapp.com/v2/sources. When trying to use a source that's not valid, the API will now return a HTTP status code of 404.

[Kamaropoulos]

Thanks a lot @ExpDev07, you're awesome!