Add a scanning option to skip the initial input enumeration

[bradlarsen]

noseyparker scan currently always does an initial enumeration of the filesystem inputs. The only user-facing reason for doing this currently is to show a progress bar when scanning. This is detrimental in a couple cases:

1. When running without a terminal (like with output directed to a file), the progress bars are not shown, and the initial filesystem enumeration is not useful.
2. When scanning large inputs from slow filesystems (like old magnetic disks or Docker bind mounts on macOS), simply enumerating the inputs can be as slow as actually scanning everything!

It would be useful to automatically avoid enumeration when progress bars are not displayed. It would also be beneficial to have an explicit control to avoid enumeration, to help in cases of slow I/O.

[bradlarsen]

This is mitigated by #216: now, there is no initial input enumeration at all.