Consider adding lib/semver module

Not quite the same as #119, though it may end up being related. More specifically, I'm thinking of generalizing the [semver](http://semver.org/) parsing and checking logic from [scripts/test.d/browser in mbland/custom-links](https://github.com/mbland/custom-links/blob/e1e6a656ca4e64842200472e964f9054e1df8115/scripts/test.d/browser#L113-L133) introduced in mbland/custom-links#139.