Clean up pipermail attachments from email.content

When loading an archive from pipermail, attachments are
described in the email body as "-- next part --" pr
attachment.

This is noise for the reader and should not be part of the
email.content displayed to the user. The complete original
message can still be seen in the raw view.
Merge pull request #3 from aslakknutsen/remove_next_part

Clean up pipermail attachments from email.content