Proposal: copy attachments

[robtsah]

I'm using slack-export-viewer on a remote server because my Slack export archive is too large to process locally.

I run the slack-export-viewer command to generate the output directory, which I serve as a static website using Nginx.

Problem:
Files and attachments (such as images, uploaded documents, etc.) are not copied into the output directory during export. As a result, links in the generated HTML point to files that do not exist locally, making attachments inaccessible when browsing the exported site via Nginx.

Expected behavior:
Attachments referenced in the exported conversations should be present in the output directory and accessible from the static site.

I get this:

[bbendick]

This is "half" covered by this PR. #200 It modifies the HTML, but doesn't copy the file.