Feature: option `--ignore` to skip certain directory/directories

[cuducos]

Hi, I loved the app. Thank you so much 💜

I was wondering if we could have the option to skip certain directories. I am happy to try to come up with a draft PR if that's an interesting thing to have

[gl-yziquel]

I have the exact same requirement. I'd like to leave out certain mount points when running diskonaut on /.

The ncdu tool has an -x option that disable filesystem boundary crossing. An -x option for diskonaut would be perfect.

[w00dst0ck]

+1

[seanmorton]

dupe of #80