Add a --add-host host:ip flag which appends lines to /etc/hosts.

As per https://github.com/dotcloud/docker/issues/2335 and https://github.com/dotcloud/docker/issues/2267 and maybe others, this adds a simple flag to append lines to /etc/hosts.

This is my first foray into go and Docker code, so I'm happy to be told where I screwed it up.  I mostly followed the example of the Env flag.