patman: Add --thread option

Add option to create threaded series of patches.
With it, it will be possible to create patch threads like this:
[PATCH 0/10] Add support for time travel
  [PATCH 1/10] Add Flux Capacitor driver
  [PATCH 2/10] Add Mr. Fusion driver
  (...)

Internally it will call git send-email with --thread option

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>