Thunderbird plugin

[zandr]

It would be very cool to be able to use Tahoe to transmit file attachments in email.

The idea is that we'd have a Thunderbird (and maybe Mail.app? or <gasp> Outlook) plugin that provided an alternate 'attach' button.

Clicking said button would allow a user to select a file or files. The 'attach' button in this dialog would cause the following:

Upload selected files to the mesh. Append the tahoe URL to text of the email message.

This *probably* requires that we implement sharing slots as described in #152, since the user won't want to be blocked waiting for the upload.

Designing a postfix filter to accomplish the same function is left as an exercise for the reader.