Add 8bit Content-Transfer-Encoding support.

Signed-off-by: Dmitry Bogatov <KAction@gnu.org>
Add use of `sender' parameter in `sendmail_send'.

Signed-off-by: Dmitry Bogatov <KAction@gnu.org>
feed: Pass config and section arguments to get_message()

Otherwise it will always use the default config.

Also add section fallback code to get_message in case the
feed-specific section is not in the config file.  This is useful for
testing, although in production every feed should have a section to
hold it's URL.

Signed-off-by: W. Trevor King <wking@tremily.us>
config: Rename use_8bit to use-8bit for uniformity

We use hyphens in all the other config settings.

Signed-off-by: W. Trevor King <wking@tremily.us>
test/bbc-chinese: Add tests for use-8bit

The BBC's Chinese feed should have a few non-ASCII characters in it
;).

Signed-off-by: W. Trevor King <wking@tremily.us>
email: When setting an 8bit CTE, remove the old header

From the docs [1]:

  Note that this does not overwrite or delete any existing header with
  the same name. If you want to ensure that the new header is the only
  one present in the message with field name name, delete the field
  first, e.g.:

    del msg['subject']
    msg['subject'] = 'Python roolz!'

[1]: http://docs.python.org/3/library/email.message.html#email.message.Message.__setitem__

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