Add option to print totals

[vladh]

This PR adds the --totals option, which makes duf print the total size, usage and free space for every group in a table footer, as well as an extra small table at the bottom with the overall totals. It looks like this:

[vladh]

@muesli Would you like to see any changes here? Just checking in for an update. Thank you!

[muesli]

@vladh Apologies that I haven't found the time to review this yet. It's overdue!

[vladh]

No worries! I've fixed the style issues. Let me know if you'd like me to rebase into one commit.

[vladh]

@muesli Just wanted to ping you again on this, let me know if you need any changes!

[vladh]

@muesli One last ping from me! No stress, hope you're doing well. :)