support for promises in up/down functions

[stalniy]

I'm using nodejs 6.9+ and would like to use built-in Promise libraries instead of done callback. So,

module.exports = {
  up() {
     return this.db.createCollection('tasks')
  },

  down() {
     return this.db.dropCollection('tasks')
  }
}

[stalniy]

In future it will also support async functions and I'll be able to do this:

module.exports = {
  async up() {
     await this.db.createCollection('tasks')
  },

  async down() {
     await this.db.dropCollection('tasks')
  }
}

[KieronWiltshire]

#65