Add command-line option or separate capture tool

[mossmann]

It should be possible to save a live capture to a file without the GUI.

[martinling]

On Windows, this might have to be a separate binary, because as far as I'm aware it's not possible to have an executable that's a GUI application (i.e. can run without opening a console window), but also outputs to the console if run from the command line.

This is confirmed by Raymond Chen here, although there are some ways to fake it listed in this stackoverflow thread.

[mossmann]

I think the typical solution is to have an additional executable like packetry-cli which I would be fine with.

[martinling]

Yeah, but that reintroduces all the pain we had managed to remove in #105.