Async hook support

[kruschid]

I would like to use bcrypt in order to encrypt user passwords. Basically bcrypt provides both synchronus and asynchronous methods to achieve this.

Regarding the performance I tried the async method inside a mutation pre hook first:

mutation: {
  pre: (next, args, context, info) {
    bcrypt.hash(args.password, 8, (err, hash) => {
      args.password = hash;
      console.log(args.password);
      next(args, context, info);
    });
  }
}

Here console.log(args.password) prints the hash on the console but my query returns a user object with the raw password instead the hash. Calling next(args, context, info) inside the callback has no effect!

In #123 I found a working mutation hook example so I changed the code above using bcrypt's sync function.

mutation: {
  pre: (next, args, context, info) {
    args.password = bcrypt.hashSync(args.password, 8)
    next(args, context, info);
  }
}

Now the same query returns a user object with the hashed password.

mutation {
  updateUser(input:{
    id: "a00000000000000000000001"
    username: "name"
    password: "password"
  }){changedUser{username,password}}
}

The user object:

{
  "data": {
    "updateUser": {
      "changedUser": {
        "username": "name",
        "password": "$2a$08$guy.E3lD7P2Yov3gTSUvnurxK/XN7KgjFjTrlLmXPL6IElbBVLukO"
      }
    }
  }

I can't imagine that this is intended since there could be a great demand for async hooks.

Node v4.5.0 Graffity-Mongoose v5.3.0