platform: Show platforms that are out of date

http://mail-archives.apache.org/mod_mbox/cordova-dev/201312.mbox/%3cCAEeRA4o6NNKXXtvmq=O9jZsFnTZjajLf9KcwQQn+1Oc+7ifhmw@mail.gmail.com%3e

Is a discussion about warning that platforms are out of date.

(JIRA is currently down)

For now, I just want a command which users (or tools) can use to determine if their platforms are out of date.

From there, we can bike shed how often it gets run.