email: Stub out send_imap()

email: Stub out send_imap()

Arun Persaud suggested IMAP as an additional email delivery mechanism.
The benefit of using IMAP over SMTP is that you can set the target
mailbox directly (instead of filtering the incoming mail with procmail
or a similar external tool).  This commit restructures the 'send'
configuration to support IMAP output with a configurable mailbox.
That means you can do something like:

  [DEFAULT]
  email-protocol: imap
  imap-auth: True
  imap-username: myname
  imap-password: mypass
  imap-server: imap.yourisp.net
  imap-port: 993
  imap-ssl: True

  [feed.rss2email]
  url = http://www.allthingsrss.com/rss2email/feed/
  imap-mailbox = rss2email

  [feed.xkcd]
  url = http://xkcd.com/atom.xml
  imap-mailbox = xkcd

For non-IMAP users, note that the boolean `use-smtp` configuration
variable is gone, replaced by the more flexible `email-protocol`.
You'll want to replace:

  use-smtp = False

with:

  email-protocol = sendmail

and replace:

  use-smtp = True

with:

  email-protocol = smtp

Signed-off-by: W. Trevor King <wking@tremily.us>