`magit-section-jump-map': quickly jump to sections

Add a `magit-status-mode-map' submap which allows to quickly
jump to some common sections in `magit-status-mode'.

Fixes #919.

Suggested-by: Kristian Lein-Mathisen <kristianlein@gmail.com>
Signed-off-by: Pieter Praet <pieter@praet.org>