Adjust json to point to downloaded attachments

[kw217]

Hi - thanks for this awesome tool!

I am downloading a private channel which contains a lot of images (graphs). I'm using python slack-export-viewer to view the resulting ZIP file. Even after downloading attachments, the links and inline images still point to files.slack.com rather than into the archive.

Suggestion: slack-advanced-exporter should rewrite the links in JSON to point to the downloaded attachments, so that slack-export-viewer displays them.

Thanks.