Add ability to generate multipart/alternative using a filter script.
Create config variables $send_multipart_alternative (a quadoption)
and $send_multipart_alternative_filter. The filter script expects
output to be the mime type, a blank line, then the content.
Add ability to preview alternative from compose menu. <view-alt-text>
forces viewing as text. <view-alt-mailcap> forces using the mailcap.
Bind them to 'v' and 'V', respectively.
Improve <resend-message> so a multipart/alternative is stripped out,
which would just confuse the compose menu. Currently this preserves
the first alternative.
Allow alternative generation in batch mode, as long as the quadoption
is set to 'yes'.