zsh: update zsh completion for docker command

zsh completion is updated with the content of
felixr/docker-zsh-completion.

The major change since the last merge is the removal of use of
sed/awk. This should help a lot OSX users who previously had to install
gawk and gsed.

Docker-DCO-1.1-Signed-off-by: Vincent Bernat <vincent@bernat.im> (github: vincentbernat)