docker save: --output flag

for those that do not care to redirect stdout

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
docker save: add and improve docs

add usage examples for `docker save ...`

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
api/client: var style tweak

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #4778 from vbatts/vbatts-save_to_output

`docker save --output` flags