sniffer: make serial/socket command optional

[miri64]

First of all: I know this is very hacky, but sadly there is no straight-forward (and portable) way with argparse to make a subcommand optional, like it is the case e.g. with many git commands.

However, this allows now also do just type

./tools/sniffer /dev/ttyACM0 112500 26

Depends on #37

[smlng]

please squash

[smlng]

and rebase after #37 was merged.

[smlng]

would prefer #40 though

[miri64]

Ok