Feature: option to include free disk space

This may only make sense when scanning whole mounts, but it would be nice to have a visual feedback no how much free space there is. It could start as something as simples as a special node/branch that accounts for what `df` would returnn on the given path. Like `diskonaut /foo/bar` would include a special colored "free space" with the size of `df /foo/bar`.

Start from the `diskonaut /foo/bar` command path and trace how mount contents are scanned and represented in the terminal view. Compare the target path with `df /foo/bar`, then define how a distinct colored free-space node or branch would fit the existing visualization. Done means the view shows free space with the size reported for the given path.

Written by the indexing model from the issue text.