Allow declaring a common root for routes.

[Jezza]

It would be cool if we could, when declaring routes, specify a context for a given set of routes.

It's not annoying per se, but it would be nice to see some structure.

On.context("/context", () -> {
    On.get("/thing1").json(...);
    On.get("/thing2").json(...);
})

[Jezza]

I just realise it could be given the setup to setup the routes.

On.context("/context", setup -> {
    setup.get("/thing1").json(...);
    setup.get("/thing2").json(...);
});

EDIT: But then I just realised the setup is fairly static, so we can't just alter that...