stg mail: add basic support for git send-email

stg mail: add basic support for git send-email

This is the first step in turning stg mail into a wrapper for
git send-email. It requires passing the --git option to stg mail
for now.

Only a few basic options are supported for now, namely To/Cc/Bcc.

git send-email options used:
  --suppress-cc=self	prevent further information prompts
  --quiet		reduce git send-email output

Cc: Karl Wiberg <kha@treskal.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>