Add docker remote sub-command.

Add the docker remote sub-command (actually the _docker-remote script
but can be invoked as a sub-command with the wrapper script). It creates
a local socket that's forwarded to the dockerd socket on the remote host
via ssh. Requires OpenSSH 6.7 or later (for Unix socket forwarding and
%C token. Typical invocation would be `$(docker remote
host.domain.example)`. A blog post will follow.

Documents/bin/_docker-remote

+#!/bin/sh
+set -eu
+
+[ -n "$1" ] || { echo "You must specify host." >> /dev/stderr; exit 1; }
+host="$1"
+localsocket="$HOME/.ssh/docker_$host.sock"
+ssh="ssh -fnNTS ~/.ssh/%C.sock"
+
+# Check for exiting forward
+if ! $ssh -O check -L "$localsocket:/var/run/docker.sock" "$host" 2> /dev/null
+then
+
+    # Delete socket if it exists
+    [ ! -S "$localsocket" ] || rm "$localsocket"
+
+    # Open SSH ControlMaster
+    $ssh -o ExitOnForwardFailure=no -o ControlMaster=auto -o ControlPersist=0 "$host"
+
+    # Forward socket
+    $ssh -O forward -L "$localsocket:/var/run/docker.sock" "$host"
+fi
+
+# Echo the DOCKER_HOST export command, for eval'ing.
+echo "export DOCKER_HOST=unix://$localsocket"