Refactor context middleware

[mercs600]

We should deliver possibility to override middleware https://github.com/TYPO3-Initiatives/nuxt-typo3/blob/master/lib/templates/middleware/context.js#L38 in an easy way.
Additionally we should deliver own hooks (https://nuxtjs.org/docs/configuration-glossary/configuration-hooks) to override context and page response in specific cases.

potentially places to use hooks:

1. before server/client redirect

if (response.data.redirectUrl) {
      if (process.server) {
        return context.redirect(
          response.data.statusCode || 301,
          response.data.redirectUrl
        )
      } else if (process.client && typeof window !== 'undefined') {
        window.location.href = response.data.redirectUrl
        return context.next(false)
      }

for example you may want to return specific headers with this redirect response.

2. before setup page response

context.pageContent = response.data
context.store.commit(SET_PAGE, response.data)

to override it in specific cases.

hooks should gives context and current page slug as a parameters
it should be documented with examples