Consider enabling callbacks provided to `WithRemoteData

[oyvindberg]

    Would  it make sense to have `onPending` and `onError` callback props as well?

Originally posted by @jestho in #2 (comment)

[oyvindberg]

It would make a lot of sense, but I'll defer the decision.

The () => Promise<T> function passed to useRemoteData is currently the place to put those callbacks, and there is some value in having few ways of expressing the same think.

Should at least go in documentation though.