Skip push cli option

Adding `--skip-push` option to `builder build` cli command per https://github.com/modcloth/docker-builder/issues/45

Saves from having to edit the `Bobfile` setting `skip_push = true` every time you want to do local testing

Merge after https://github.com/modcloth/docker-builder/pull/43 and https://github.com/modcloth/docker-builder/pull/44