Add gitlab-shell#path option in config

Before this commit gitlab-shell but me placed directly in home dir.
Ex:
  /home/git/gitlab-shell

After this change you can place gitlab-shell in custom location.
Ex:
  /Users/developer/gitlab/gitlab-shell

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>