Feature: Check before pull

[dlutsch]

I'm using docker-mirror to manage a very large number of docker images. If I run this on a new machine, each image needs to be pulled and re-tagged regardless of whether or not that image tag already exists in the target mirror repo. It would be great if there was a config option to change this behavior and have docker-mirror check the target repo first to validate whether the proposed tag exists first. This way it would only pull the image from the source repo when the tag doesn't exist in the target. This saves time and disk space, and avoids the issue of dockerhub pull rate-limiting.

I'm not sure whether SHA validation would be necessary or not, but it could be achieved if the arch was also added to the config, an env var, or otherwise read from the system.

[josegonzalez]

Pull requests welcome :)