Selective attachments download (i.e. only private/only public)

[rusq]

Note it might as well be useful for slackdump to allow for the download of all attachments or only these present in private channels/DMs - not these present in public channels. Maybe via a flag like -download [all (default) | private-only | public-only]

Originally posted by @jo-chemla in #232 (comment)

[rusq]

Partly Implemented in v3 via search files command, that allows for arbitrary search terms