Split attachments folder into subfolders

[ReagentX]

This came from feature request #51.

Right now all attachments get copied to output_dir/attachments per:

https://github.com/ReagentX/imessage-exporter/blob/release/imessage-exporter/src/app/runtime.rs/#L77-L82

When we copy to that location, all we do is drop in the file with a unique name:

https://github.com/ReagentX/imessage-exporter/blob/release/imessage-exporter/src/exporters/html.rs/#L411-L421

It would be better to have separate subdirectories for each conversation to make it easier to find attachments manually.