[Mailman-i18n] Add header/footer to MIME multipart (was: the way i added my language charset)

[Barry A. Warsaw]

    | Another option for MIME multipart messages is to embed the
    | message parts into a _new_ multipart/mixed structure which
    | contains
    | 1st the header as text/plain, then
    | 2nd all parts of the message itself, then
    | 3rd the footer as text/plain.

I thought about this when the new MIME-aware Decorate.py code was
written, and it's definitely a possibility.  If you look at the code,
you'll notice some placeholders where this could be added.

My main reservation about doing this is about the best way to craft
this new outer multipart/mixed message.  In all the other cases, we're
basically keeping the outer chrome intact, e.g. the headers from the
originally sent message are basically passed through.

But in the above approach, we'd have to wrap the original message in
one of Mailman's own creation, so what do we use for things like the
To, From, and Subject headers?  Do we pass through Received headers,
References header, etc?  Those were all issues that I couldn't decide
on, so I decided to punt for now.

-Barry