Proposal: Add version diff

[gdavison]

go-version currently supports comparing versions as a whole, e.g. 2.3.4 > 1.2.3. Sometimes we need to determine if the difference between versions is a major change, a minor change, etc.

Since Compare() supports an arbitrary number of components, the diff should as well.

Only the first diff is likely to be relevant, as in the diff of 2.3.4 vs 1.2.3, we care about the major version change, and the minor version changes are not meaningful.

I can work on this if this sounds good. We're going to be using a very pared-down version of this in the AWS Provider.