Feature Request: Option to set fixed unit for network speed

[hexaholic]

Since #52 it is possible to switch between bits/second and bytes/second, which is a great improvement. But the displayed units will still change in their magnitude, e.g. between Kbps, Mbps etc, depending on current traffic throughput.

Please add an option to set the unit to a fixed value. For example, I would like to set it to always display Mbps but never Kbps, Gbps and so on.

[Croydon]

I want to switch to OpenNetMeter from NetSpeedMonitor, which is closed-sourced and abandoned.

This would be the feature that I would probably miss the most right now. I would like to see a multiple-choice for a fixed unit