[changed] activeRoute can render with props and children.

Potentially fixes #59.

`{this.props.activeRoute}` is now `{this.props.activeRoute(props, children)}`.
/facepalm. Yes we do. I've discovered that we also need to return a function when activeRoute is null, or in practice you need to do `{this.props.activeRoute ? this.props.activeRoute() : null}` which is annoying as hell. Both fixes on the way.