Add option to lookup image version for tags with latest

[rgstephens]

For updates to images with latest tag, it would be nice to find the version number associate with this image.

If the findVersion option is specified, search recent images that have the same digest value and report the associated image tag. For example, if there's a digest value identical to latest with the tag 3.2.0, the message would report that 3.2.0 was updated instead of latest.

Along with this would be a versionMatch option. If this option is specified, then the update will only be reported if there's tag matches that pattern. In the example below, a tag off 3.2.0-beta would not match and as a result the notification would not be sent.

    "notifyServices": [
        {
            "image": "rasa/rasa:latest",
            "findVersion": true,
            "versionMatch": "^\\d{1,2}\\.\\d{1,2}\\.\\d{1,3}$",
             ...

[rgstephens]

A docker image build with this feature can be pulled at stephens/docker-notify:1.4.1

[tonycody]

Let's merge them