Read published addresses from SERVICES_EXT env var

Additionally to registering all services exposed by Docker, registrator
will now also register addresses specified in SERVICES_EXT.

This is useful if you're using host networking and the container manage
it's addresses on it's own. This closes #77.
Merge pull request #1 from discordianfish/77-add-host-networking-support

Read published addresses from SERVICES_EXT env var