Provide additional ctx for RenderToken

[revskill10]

We should provide additional ctx for the custom renderer could have additional control logic.

[lhorie]

I actually feel like this could/should be looked at separately from #285, which is a bigger change

[lhorie]

Rationale: non-react view libraries might not expose a context-like API, so there needs to be some way of determining request data from render for e.g. routing purposes. Custom renderers may also need to have access to ctx for various reasons (e.g. streaming rendering)

[revskill10]

@lhorie Could you give me one example of that use case when we need to check ?

[lhorie]

For example, if you want to conditionally render from a set of components based on the url, or based on the existence of a cookie