Telemetry log endpoint feature

[landswellsong]

I've added an option to specify tlog: endpoints that would write incoming traffic in a Mavlink Logfile.

I thought it might be useful to make a PR of. It seems to be working alright, but my Go is very rusty (pun not intended) so I think the quality of the code can be improved.

In particular things I'm not very sure about:

• Is it fine to lock up the Read() method like that? Ideally I'd pause that goroutine until the exit sequence, but I'm not very sure how to do that properly without altering gomavlib too much.
• Does gomavlib issue writes in full packets? There seem to be some buffering involved, but I'm not entirely sure how Go's buffering operates. Currently my code assumes that the bytes to write is a whole single packet.