[added] Support custom RoutingContext on Router

[taion]

Closes #2088

Another attempt at #2376. The tests illustrate the intended usage. I think this is a much less gross API on the React Router side.

[taion]

My intention here on the react-router-relay side then is to expose:

• RelayRoutingWrapper
• RelayRoutingContext
• RelayRouter

The code would be something like:

const RelayRoutingContext = ({forceFetch, ...props}) => (
  <RelayRoutingWrapper {...props} forceFetch={forceFetch}>
    <RoutingContext {...props} />
  </RelayRoutingWrapper>
) 

const RelayRouter = (props) => (
  <Router {...props} RoutingContext={RelayRoutingWrapper} />
);

The idea is that most users can just use RelayRouter and RelayRoutingContext, while somebody who needs to can build their own custom RoutingContext with RelayRoutingWrapper and whatever other wrappers they need.

[taion]

Done making trivial tweaks to the new test cases... finally.

[timdorr]

(nit) This should be camelCased as routingContext.

[taion]

Works for me - I've seen people do both for custom component classes depending on the context and don't have a personal preference.

[timdorr]

I suppose it would be different if the DI here was done via module. But then you wouldn't even have this problem in the first place (you'd have others, but that's a different story...)

[taion]

Implicitly I'm sorta re-using the module name here: https://github.com/rackt/react-router/pull/2383/files#diff-1063d4db6b1a5033d4afb74421a91ef4R31

It's not extremely clever, but it saves a tiny amount of typing.

[taion]

@timdorr I don't mind changing this to routingContext. But since you're looking at this - does this make sense as an API entry point? This is pretty much resurrecting #1870, which we abandoned at the time but I think I need it at this point.

[taion]

@timdorr Updated with your feedback.

I also realized that I have to pass extra props from Router into the custom routing context for things to work nicely, and I think it's reasonable to drop routes as a prop from route components with this PR.

[timdorr]

Could you not communicate that information via context? Or could you not do what redux-router does and provide your own RelayRouter top-level component?

[taion]

@timdorr I can - however, the only thing I need to override really is just the RoutingContext used. I could copy/paste the entirety of Router, but it seemed like it'd be cleaner if I could just implement RelayRouter as <Router {...this.props} routingContext={RelayRoutingContext} />.

[taion]

@mjackson @ryanflorence

If either of you have a chance, I'd appreciate it if you could have a quick look at the proposed API (i.e. adding a routerContext prop to Router) and let me know if this is a reasonable approach, or if I need to go back to the drawing board again.

This isn't relevant for 1.0.0, but it'd be helpful to know if this approach seems reasonable.

[taion]

Shoot, if we want to drop routes as a prop from route component, this actually can't be for 1.1.

[ryanflorence]

Keep this.props.routes

And then lets merge this.

[taion]

Squashed, rebased, renamed routingContext back to RoutingContext. The test coverage here exercises everything I care about.