Render: Presenter

[VojtechVitek]

Present your data models and/or create API object versioning!

func handler(ctx context.Context, w http.ResponseWriter, r *http.Request) {
    obj := &Object{
        ID:       1,
        AuthData: "secret data for auth'd users only",
        Data:     []string{"one", "two", "three", "four"},
    }

    render.Respond(ctx, w, obj)
}

v2 := render.NewPresenter()
v2.Register(func(ctx context.Context, from *Object) (*v2.Object, error) {
    to := &v2.Object{
        Object: from,
        URL:    fmt.Sprintf("http://api.example.com/objects/%d", from.ID),
        Data:   map[string]bool{},
    }
    for _, item := range from.Data {
        to.Data[item] = true
    }
    return to, nil
})

r.Get("/", handler) // renders raw Object
r.Get("/v2", render.UsePresenter(v2), handler) // renders Object in version 2

[VojtechVitek]

This copies Presenter conversion functions from other presenters.. Should we call it .Copy()?

or maybe pass it to NewPresenter() func:
version1 := NewPresenter(version2, versionLatest)

[pkieltyka]

this is pretty cool, I like the short-hand

[VojtechVitek]

Closing this PR and will open new one against v2.