Adding req.matchedVersion()

When a route is defined as responding to multiple version numbers then if the client supplies a version range that it can accept such as `accept-version: <2.2.0` it can be useful for the route to know exactly which version number from its array of supported versions matched the clients version specification.

req.version() already exposes the clients version string but it is often a range such as <2.2.0 or 2.*
req.matchedVersion() allows a route which matches versions 2.0.0, 2.1.0, and 2.2.0 to know that a client who specified `accept-version: <2.2.0` reached the route because 2.1.0 matched the request header.

This pull request includes documentation, and a test.
```
if (x) {

} else {

}
```