Add support for Virtual Radar Server

[wiseman]

I'm working on adding support for using Virtual Radar Server instead of dump1090. VRS has more options for the map display, and already has things like integration with airport-data.com to show photos of aircraft, or with a database of registration info.

[wiseman]

Also relevant: #12

[kevinabrandon]

@wiseman a few days ago I accepted your pull request #29. I'm finally testing it now but it looks like you may have forgotten to add a file datasource.py? Can you take a look?

[wiseman]

Criminy. Sorry about that! See PR #32.

[kevinabrandon]

Thanks 👍