send-email: Allow the envelope sender to be set via configuration

Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -209,6 +209,7 @@ sub format_2822_time {
     "bcc" => \@bcclist,
     "aliasesfile" => \@alias_files,
     "suppresscc" => \@suppress_cc,
+    "envelopesender" => \$envelope_sender,
 );
 
 # Handle Uncouth Termination