support rtl_tcp as an input source

This was previously requested in #87. The problem with the solution suggested there is that it requires starting `rtl_tcp` with a specific set of parameters, and the problem with that is that the running configuration of `rtl_tcp` can be changed remotely using, for example, tools like `gqrx` or really anything using the `osmocom_sdr` source in gnuradio.

Ideally, it would be possible for `dump1090` to connect directly to a remote `rtl_tcp` instance and tune it to the correct frequency.