implement duplicated field detections

[vvakame]

https://facebook.github.io/graphql/draft/#sec-Objects

3.6 Objects
Type Validation
The field must have a unique name within that Object type; no two fields may share the same name.

Object Extensions
Type Validation
The fields of an Object type extension must have unique names; no two fields may share the same name.
Any fields of an Object type extension must not be already defined on the original Object type.

---

  type Query {
    hello: String
    hello: String
  }

error on graphql-js!
https://runkit.com/vvakame/5bf29d85574142001208b7d5

---

  type Query {
    hello: String
  }
  extend type Query {
    hello: String
  }

🤔 ❓
https://runkit.com/vvakame/5bf29df977f07a001281a18a

---

  type Query {
    hello: String
  }
  extend type Query {
    bye: Int
    bye: Int
  }

🤔 ❓ ❓
https://runkit.com/vvakame/5bf29db24a9ab200128a6c24

[vvakame]

BTW, parser can't validate __foobar field now.

All fields defined within an Object type must not have a name which begins with "__" (two underscores), as this is used exclusively by GraphQL’s introspection system.

[coveralls]

Coverage increased (+0.01%) to 90.995% when pulling 9c02695 on feat-duplicate-check into c336811 on master.