git-send-email: make options easier to configure.

git-send-email: make options easier to configure.

This change makes git-send-email's behavior easier to modify by adding config
equivalents for two more of git-send-email's flags.

The mapping of flag to config setting is:
  --[no-]supress-from => sendemail.suppressfrom
  --[no-]signed-off-cc => sendemail.signedoffcc

It renames the --threaded option to --thread/--no-thread; the
config variable is also called sendemail.thread.

Signed-off-by: Adam Roben <aroben@apple.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>