Added app.override() to create plugin inheritance chain

[mcollina]

cc @delvedor

[delvedor]

LGTM, if you want you can add the following test case for override, which is specific for the fastify use case.

test('fastify test case', (t) => {
  t.plan(7)

  const noop = () => {}

  function build () {
    const app = boot(server, {})
    app.override = function (s) {
      return Object.create(s)
    }

    server.add = function (name, fn, cb) {
      if (this[name]) return cb(new Error('already existent'))
      this[name] = fn
      cb()
    }

    return server

    function server (req, res) {}
  }

  const instance = build()
  t.ok(instance.add)
  t.ok(instance.use)

  instance.use((i, opts, cb) => {
    t.notEqual(i, instance)
    t.ok(instance.isPrototypeOf(i))

    i.add('test', noop, (err) => {
      t.error(err)
      t.ok(i.test)
      cb()
    })
  })

  instance.ready(() => {
    t.notOk(instance.test)
  })
})