clear error message on UnknownHostException
Add opml2clj command
Catch ParsingFeedException in feeds2imap.feeds/parse
Catch IllegalArgumentException in feeds2imap.feeds/parse
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

  * Now we can see feed's title and link in place of a "real name" in the From: header.
  * Entries within the same IMAP folder are easier to sort and organize.