Invoke a hook before/after generating a page #89

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Feature request

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What problem does this feature solve?

Would be useful for https://github.com/egoist/saber/issues/71

What does the proposed API look like?

api.hooks.afterGeneratePage.tapPromise(ID, context => {
  const amp = shouldGenerateAMPPage(context)
  if (amp) {
    //...
  }
})

How should this be implemented in your opinion?

N/A

Are you willing to work on this yourself?

PR welcome.