Allow configuring which disks/mount points to show on screen

[andrewheberle]

As I could not get any disk apart from the root filesystem to show on the OLED screen and it seemed based on my reading of the current code that this would never show anything except the root in it's current state I put together this PR.

This PR adds the ability to list up to four additional drives/mount points to display on the OLED screen as follows:

[disk]
media = md0
home = sdb1

The above config would show the free space for the filesystem on /dev/md0 and /dev/sdb1 on the screen along with the root filesystem similar to the following:

Disk:      root  8%
media 25%  home 18%

The root filesystem free space is always shown, and honestly the display of three or four additional devices means the text is quite small, but it at least gives this option.

This PR would fix #6