Validate "or later" licenses

The `master` branch implementation treats "or later" licenses as separate keys with an eventual aliases.

The `alternate-or-later-handling` [branch](https://github.com/nexB/license-expression/tree/alternate-or-later-handling) implementation treats "or later" as keywords and not as separate license keys.
If this later implementation ends up a winner, if would make sense to add validation to license symbols to check if a license supports an "or later" version or not to avoid stupid things like "MIT or later"