kanso publish should warn if the version in kanso.json has already been published

[ryanramage]

A good practice for module publishing is not to publish the same version. A couple of options:

1. Issue a warning when doing a kanso publish if the version of kanso has already been published
2. Have another field in kanso.json called version-status which if exists could have the following values: snapshot, release. If the value in the existing version is snapshot, proceed. if the status in the existing version is release, do not allow the publish.