Add request promise resolver

[simenbrekken]

This adds a new module resolver which is capable of resolving promises recursively without an external context.

Usage example:

const renderApplication = () => renderToString(<Application />)

try {
  const markup = await resolveRequestPromises(renderApplication, { maxIterations: 2 })

  res.send(markup)
} catch (error) {
  next(error)
}

Closes #2 and #3

This still needs to be documented properly along with examples for which I've created a separate issue #5

[einarlove]

Why export registerRequestPromise? What is the use-case?

[simenbrekken]

Agreed, removed in f1b52fd

[einarlove]

Why not

registerRequestPromise = promise => {
  if (resolving && promise) {
    promises.push(promise)
  }
}
// fetchData.js
componentWillMount() {
  registerRequestPromise(this.dispatchIfNeeded(this.props))
}

[simenbrekken]

Because the promise shouldn't be created unless we are actually resolving.still create a promise even if it shouldn't