Skip override

[delvedor]

In the previous PR (#9) we added the ovverride support to create a plugin inheritance chain.
Sometimes, for some reason, the user needs to disable the override if he has set one.

With this PR we support that.

Example:

const server = { my: 'server' }
const app = boot(server)

app.override = function (s) {
  return Object.create(s)
}

first[Symbol.for('skip-override')] = true
app.use(first)

function first (s, opts, cb) {
  // some code
  cb()
}

If this is 👍 for you, I can proceed to update the docs.

[mcollina]

docs :).