Handle HTML emails and other attachments

[simonw]

This has only been tested against plain text emails so far.

I think HTML emails should have the HTML stashed in a separate column.

Not sure how best to handle other attachments though, especially attachments that might be images included in the HTML.

Should also think about XSS attacks and suchlike: should this tool run HTML through https://bleach.readthedocs.io/ or should tha be left as an exercise for the user?