Accept disk path(s) to display usage of specified disk / multiple disks

[Rolv-Apneseth]

I have most of the code for this feature done, but I'll wait for the libmacchina crate to be updated to 8.0.0 so it can accept the paths.

My current approach is to add a new option called disks which can be used like this in the config:

disks = ["/", "/home/user"]

Or like this as an argument:

--disks '/,/home/user/'

Note that the show option of DiskSpace would still be required, as otherwise disks won't do anything. I did it this way to avoid a rewrite of how the show values are currently handled. The other way I thought of doing this was making show into an array of tables but that seems overly complicated and it's a breaking change to everyone's configs. What do you think of the approach I took / did I maybe miss a simpler approach?