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

[Rustam]

          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)