use tabwriter to display usage in mflag

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
improve some usages

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
update the docs to reflect the nice \n handling

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@fosiki.com> (github: SvenDowideit)
Merge pull request #4 from SvenDowideit/improve_mflag_usage_docs

update the docs for the new \n rendering
Merge pull request #5634 from vieux/improve_mflag_usage

Add support for \n in usages