Router examples

It needs to be settable by the package user and used within the package (see [here](https://github.com/flynn/flynn/pull/548/files#diff-2903e4f83baacf364b9b5ee0025aaa77R28) and [here](https://github.com/flynn/flynn/pull/548/files#diff-220e3b0d236b543194852974a777f09eR112)).