Feature request: Archival Plugin

It would be very good to have a Plugin for archiving content on the local hard disk:

*Usage example*: ZeroMail has the downside that if the person who has sent me the mail needs space, he will delete his message that will shortly after deleted also from my account.

If we had such a plugin, the sites could ask the permission to use some local HD space to save every mail that I receive and only delete them if I want to.

No files, tests, or entry points are named. Start by reviewing ZeroNet's plugin architecture and the existing mail storage and deletion flow. Done should include a permission-based local archive for received mail that preserves messages until the user chooses to delete them.

索引モデルが issue の本文から書いたものです。