filters: enable operators, instead of just "="

This reworks the filter arguments, and includes an initial new operator
of `docker ps -a --filter exited!=0`

Signed-off-by: Vincent Batts <vbatts@redhat.com>
I'm not sure that it worth it to have such freedom in choosing operators, because not every combination makes sense. Maybe we can use map of predefined operators? This should simplify code too I think.