Feed title or author in the From: header, publication date in the Date: header

  * Use entry's modification or publication date in email Date header.
    It allows to sort messages chronologically, no matter when feeds are added or fetched.
  * Prepend feed title or author's name to From: email header.
    It allows to distiguish posts from different sources in the same IMAP folder.

Given `{:from "rss@bot.com"}` in `urls.clj`, most messages will appear as either

    From: Some Blog http://someblog.com/ <rss@bot.com>
    Subject: Post Title
    Date: [publication date]
    ...

    or

    From: First Author, Second Author <rss@bot.com>
    Subject: Post Title
    Date: [publication date]
    ...