lncli: add fish-completion

This is a new, hidden command that generates completions for Fish shell.
This can be piped into `source` or saved in a file to activate (almost)
perfect tab completion.

cmd/lncli/main.go

@@ -390,6 +390,7 @@ func main() {
 		deletePaymentsCommand,
 		sendCustomCommand,
 		subscribeCustomCommand,
+		fishCompletionCommand,
 	}
 
 	// Add any extra commands determined by build flags.