Add support for fetching private channels

[wetneb]

Another thing that's missing from the official Slack export is private channels and direct messages.

There is https://github.com/sebseager/slack-exporter which enables exporting them with a similar method to slack-advanced-exporter, but it's cumbersome to use it because it doesn't integrate the data it downloads in an existing dump like slack-advanced-exporter does.

It would be fantastic if slack-advanced-exporter could support this directly instead.