Set mail's Date header according to entry's update or publication date
Prepend entry's authors to email's From: header

  * change author's format to "Name <email[at]example.com> http://example.com/",
    either field is optional, separate authors with comas
  * use base64 encoding when prepending authors' string to From header
    (build an Encoded Word)

Consequences:

  * :from in `~/.config/feeds2imap.clj/imap.clj` must be just a plain
    e-mail address: no brackets, no "Real Names <rn@example.com>".
Set every entry's :authors, if missing, to feed's title and url

  * Many feeds in the wild lack :authors. Where they are present
    the messages will be attributed to their authors directly.
    Where they are not present, they will be attributed to the
    feed's source.
  * In practice many posts will appear as messages
    From: Some Blog http://someblog.com/, Subject: Post Title.
  * This allows to posts from different sources in the same IMAP
    folder, without introducing a two-level hierarchy of folders.
Merge branch 'feature/feed-title-or-authors-in-from'