Feature: auto update services

[lagartoverde]

This issue is closely related to #7.

Basically you would be able to add a label in your docker-compose file to autoupdate the services. When you run the update command we will ask docker-hub for the latest version of those images and update them if needed

Bonus point: Updates can be customized based on semi-version releases, you could add labels like 'update-patch', 'update-minor' and 'update-major', so they will only be updated on versions of this caliber or less, for example a service with 'update-minor' won't be updated from 3.0.0 to 4.0.0 but it will to 3.1.0 or 3.0.1