git-send-email: add a new sendemail.cc configuration variable

Some projects prefer to always CC patches to a given mailing list. In
these cases, it's handy to configure that address once.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -203,6 +203,7 @@ my %config_settings = (
     "smtpuser" => \$smtp_authuser,
     "smtppass" => \$smtp_authpass,
     "to" => \@to,
+    "cc" => \@initial_cc,
     "cccmd" => \$cc_cmd,
     "aliasfiletype" => \$aliasfiletype,
     "bcc" => \@bcclist,