Allow forcing resolvers on generated types

[vektah]

Fixes #189

[flicaflow]

@vektah thank you for doing this.
I'm just trying out the new feature. I noticed the the auto generated model still has the fields defined despite having a field resolver defined. Wouldn't it be more natural to have this field omitted?

[vektah]

Yeah perhaps, there are a lot of use cases for this stuff though, it might be worth a separate issue to deal with them.

some examples:

• functions in front of a primitive type eg starwars
• 1:1 / M:1 mappings where you want the id added to the model
• 1:M / M:M mappings where you probably dont want anything

[Ehekatl]

@vektah I found that force resolver doesn't work when fields have already defined in model, it that intend behavior?

e.g.

this won't work

type Host struct {
  id string
  status string
}

type Host implements Node {
  id: ID!
  status: String
}

models:
  Host:
    model: xxx/models.Host
    fields:
      status:
        resolver: true

this works

type Host struct {
  id string
}

type Host implements Node {
  id: ID!
  status: String
}

models:
  Host:
    model: xxx/models.Host
    fields:
      status:
        resolver: true