allow to unshift hooks

[bertho-zero]

Allow registering of hooks with $unshift and $push like this:

app.hooks({
  before: {
    find: {
      $unshift: someHook()
    }
  }
})

[bertho-zero]

@daffl I think similar functionality is required in @feathersjs/hooks, we may want to define basic hooks at the service level and possibly add a more global validator when integrating the service into the app.

[daffl]

I opened up a follow-up discussion for this in feathersjs/hooks#11

[daffl]

I'm going to close this for #1798 (which has passing tests now)