send-email: Add support for SSL and SMTP-AUTH

send-email: Add support for SSL and SMTP-AUTH

Allows username and password to be given using --smtp-user
and --smtp-pass. SSL use is flagged by --smtp-ssl. These are
backed by corresponding defaults in the git configuration file.

This implements Junio's 'mail identity' suggestion in a slightly
more generalised manner. --identity=$identity, backed by
sendemail.identity indicates that the configuration subsection
[sendemail "$identity"] should take priority over the [sendemail]
section for all configuration values.

Signed-off-by: Douglas Stockwell <doug@11011.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>