Introduce RouteGroup

[ahopkins]

Currently, a route is defined as defined path to reach the endpoint. Multiple methods whether in a single definition, or multiple will result in a single route.

We should add a RouteGroup that has a near identical API to the Route.

In practical example, it means that this would yield a single route:

router.add("/path", methods=("foo", "bar"))

But this would yield two:

router.add("/path", methods=("foo",))
router.add("/path", methods=("bar"))

However, they should be joined into a single RouteGroup that the router does not distinguish between when generating the source or resolving a path.