Adding attachments to outgoing mail works.

  This is a major step forward and fixes #44 / closes #44.
  Use the prompt_chars() function for send-mail confirmation.

  This avoids the need to press RETURN after composing a mail.
  Added mime_type() function which is lua-callable.

  Now use that rather than defaulting to application/octet-stream
  Updated comments on the mime_type function.
  Use an unorded_map for MIME-type lookup.

  This is cleaner and simpler to implement, although the real solution
 in the future must be to use /etc/mime.types.
  The send_mail() primitive now supports attachments.

  If you pass a table in the "attachments" key they will be
 added to the outgoing mail.