Support monorepos by coercing version strings

The updater currenty relies on `semver.valid` to determine whether incoming version strings are valid. It does this for the parsed URL as well as the tags from GitHub releases.

Monorepos have to uses more complex tag prefixes, since they can contain many projects that each have a different version. The typical format is `package-name@0.1.2`.

If the updater were to clean putative version strings with `semver.coerce`, that would solve this problem.

It looks like that would be an easy update -- if the feature is wanted I can make a PR!