remove old tag based docker hub tags

currently `hubtag` will add an automated build tag. however, as @lalyos figured out, we also want to deleted all previous version tags. this is because if they exist, any repo updates will **rebuild old tags**. if we remove the autobuild tags, the image tags will still exist, but will become immutable.

we only want to delete autobuild tags of previous github tags, and leave autobuild tags for branches alone.

also we need to figure out the best way to update latest. delete and recreate? update? we could add a simple flag `-u` to hubtag that makes it update instead of add...