Add option to see the current disk read / write speed

[mrglutton]

Is your feature request related to a problem? Please describe.
When copying network files and such over WiFi I constantly have to watch monitor to check for optimal antenna position to get higher speed. It would be more practical to just have status in traybar.

Describe the solution you'd like
To see disk read / write speeds just like we have with network.

Describe alternatives you've considered
I use System Monitor or top, but I have to watch window.