public void path(String path, RouteGroup routeGroup) {
        pathDeque.addLast(path);
        routeGroup.addRoutes();
        pathDeque.removeLast();
    }
Add a path-prefix to the routes declared in the routeGroup
The path() method adds a path-fragment to a path-stack, adds
routes from the routeGroup, then pops the path-fragment again.
It's used for separating routes into groups, for example:
path("/api/email", () -> {
....post("/add",       EmailApi::addEmail);
....put("/change",     EmailApi::changeEmail);
....etc
});
Multiple path() calls can be nested.

@param path       the path to prefix routes with
@param routeGroup group of routes (can also contain path() calls)
src/main/java/spark/Service.java