Move task list to separate command
Move task list to separate command

This keeps the semantics the same as the other commands to only list
containers, tasks, images by calling the list subcommand.

Signed-off-by: Michael Crosby <951f4c7b5d47e32d0112c78d14a98eaa23e6b5ef@gmail.com>

kunalkushwaha/containerd,dmcgowan/containerd,estesp/containerd,thaJeztah/containerd,kunalkushwaha/containerd,mikebrow/containerd,containerd/containerd,containerd/containerd,thaJeztah/containerd,dmcgowan/containerd,crosbymichael/containerd,crosbymichael/containerd,miaoyq/containerd,miaoyq/containerd,estesp/containerd,mikebrow/containerd,AkihiroSuda/containerd,vdemeester/containerd,vdemeester/containerd