User export: Allow `-download` flag to also fetch avatars

[Billiam]

Is your feature request related to a problem? Please describe.

Currently, users returned from -list-users have either gravatar, or avatars.slack-edge.com URLs for their profile avatar attributes (image_original, image_512, etc).

This makes it harder to make sure that the slack data dump is complete, since it will still rely on slack-hosted avatar images.

Describe the solution you'd like

Like other commands, it would be useful to have a -download flag to also download the avatar images locally, and return that local path information in the JSON, or use predictable naming (ex: <user_id>.png)

Describe alternatives you've considered

For my use case, I can also iterate through the images in the JSON and download them locally, but I think it would complement slackdump's other features to have this built in.

[rusq]

Hey @Billiam , that's a good suggestion, thank you. I'll add it to v3.