Add support for overriding publish platform/version

This is useful in situations where packages that are build on one platform/version are tested and released on 1..n other platforms/versions. For example, one might build on Ubuntu 10.04 and test/publish on Ubuntu 
11.04, 12.04 and Debian 7.

/cc @opscode/release-engineers 
Yes, but I kept those for BC (they existed before). I also don't think `-pv` will work - that's going to use a platform of `"v"` I think. What value is there in having the short flags?