Manual loading control

[AmpContext](https://github.com/ampproject/amp-react-prototype/blob/104861c3777d5341377092620f123b9013c0050d/src/amp-context.js#L28) allows control of rendering/loading via context properties. By default, loading can be enabled and applications can exercise a more fine-grained control using `AmpContext.Provider`. Additionally, we could standardize on a property for manual control as well to override `AmpContext`. E.g. `loading-control=auto|manual|always` or similar.