Specify destination of push

This allows you to push to any of an image's tags, not just its first, by supplying an explicit :repo_tag arg.

This will have a minor merge conflict with #222 because they both touch the top of the push method. Let me know how you want history to read and I can squash/rebase as desired.
I think the following would be a bit more clear:

```ruby
repo_tag = options.delete(:repo_tag) || info['RepoTags].first
```