docker rm -a [--all] - delete all containers

This is a simple addition to the rm command. Developers end up with a proliferation of containers when using docker,
this lets you clear them out in one fell swoop (as is part of rm command, other flags still apply.
Won't force remove unless told do, for example).

Docker-DCO-1.1-Signed-off-by: Michael Neale <michael.neale@gmail.com> (github: michaelneale)