Ability to use forceGenerate and extraFields together

[atzedus]

gqlgen.yaml:

autobind:
  - mypkg
models:
  Person:
    forceGenerate: true
    extraFields:
      IsAdmin:
        type: bool

schema:

type Person {
  id: ID!
  name: String!
}

Сurrent behavior:

type Person struct {
	ID                              string      `json:"id"`
	Name                            string      `json:"name"`
}

Expected behavior (this PR fix it):

type Person struct {
	ID                              string      `json:"id"`
	Name                            string      `json:"name"`
	IsAdmin                            bool      `json:"isAdmin"`
}

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

[coveralls]

coverage: 75.703% (-0.04%) from 75.74% when pulling c8847f0 on atzedus:force-generate-and-extra-fields into f90ac05 on 99designs:master.

[StevenACoffman]

Thanks for this and your other contributions! Nice work!