docker ps: introducing filters

* starting with filtering for exit codes. `docker ps -a --filter 'exited=1'`
* API doc for filter parameter
* formatting filters for help usage
* tweaks for review

This requires https://github.com/dotcloud/docker/pull/4430

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #6162 from vbatts/vbatts-ps_filter

docker ps: introducing filters