Please allow specifying a probe and speed when using probe-rs

[heinrichmeier]

Right now, rustmeter defaults to using the first listed probe. Some probes, like e.g. the glasgow, don't appear in probe-rs's list output (https://github.com/probe-rs/probe-rs/blob/780df7183117b1c29ec9231c6c1dc03507e90187/probe-rs/src/probe/glasgow/mod.rs#L47-L52), so they currently can't be used with rustmeter.