docker load: add --input flag

for those that do not care to read from redirected stdin

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
docker load: doc clarification

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
docker load: added example of a multiple tag image

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Merge pull request #4840 from vbatts/vbatts-load_from_input

docker load: add --input flag