Added -f/--force option to ansible-pull

I would like to use Ansible for semi-autonomous hosts which may have intermittent network connections.  These hosts are mostly stateless, except for their ansible-pull(ed) playbook repository which resides on a Flash card.  This adds a --force option to ansible-pull so that hosts such as these will run the playbook even if an update of the playbook is not possible.  Without such an option, the hosts may boot and do absolutely nothing useful.  With this option, they can continue doing what they did with their "last known" playbooks.