Support getting/reporting current values

[mrjoel]

The cli only seems to support blindly setting new values. It would be super useful to be able to get a summary of current device values to know existing settings.

My device in particular is a SteelSeries Rival 310, but this request appears to be applicable to all devices.

[flozz]

This is currently a work in progress. Since the last version (v4.4.0), Rivalcfg starts to save the current configuration on the HDD: this will be required to develop a GUI.

Please note that it is not possible to read-back values from SteelSeries mice (to be more complete, we could be able to get some of the settings (sometime with partial information only) from few devices but this will not be implemented as this is not really reliable → more info)