[RFC] Server only components

- All components are server-only when the nearest [route layout](https://github.com/alloc/saus/issues/70) has no `hydrator` defined.
- For component subtrees to opt-in to client-side rendering, they need to attach some special `data-` attributes to the subtree's root element.
- One attribute will tell Saus which component module to import and which export specifier points to the component (eg: `data-import="/path/to/module.js#MyComponent"`).
- The 2nd attribute will tell Saus which hydrator module to import (eg: `data-hydrate="/path/to/module.js"`).
- The final attribute is optional, and it will tell Saus when to hydrate (eg: `data-when="visible"`). If undefined, Saus will hydrate the subtree immediately after its modules are loaded.