SDL: Implement descriptions as strings

[jamesdphillips]

• Defined in [RFC] GraphQL Schema Definition Language (SDL) graphql/graphql-spec#90.
• Grammar Summary.
• Parser changes largely inspired by graphql-js implementation.

tldr; when describing your schema using GraphQL's Schema Definition Language (SDL) you can provide descriptions for your types using strings.

"""
My Great Type
"""
type Dog implements Pet {
  name: String!
  breed: [BREED]
}

"""
Pets are great
"""
interface Pets {
  "All pets should have a name"
  name: String!
}

"""
Dog Breeds
"""
enum BREED {
  """
  Description 1
  """
  BORDER_COLLIE
  "Description 2"
  SHIBA
  """
  Do not as me how to spell this
  """
  POM
}

// ...

[coveralls]

Coverage increased (+0.2%) to 82.236% when pulling f9aa8b9 on jamesdphillips:type-descriptions into 7731016 on graphql-go:master.

[coveralls]

Coverage increased (+0.02%) to 82.236% when pulling 1a9c9d2 on jamesdphillips:type-descriptions into 62eb652 on graphql-go:master.

[coveralls]

Coverage increased (+0.02%) to 82.225% when pulling 063a198 on jamesdphillips:type-descriptions into 41d6d3b on graphql-go:master.

[chris-ramon]

awesome!, thanks a lot @jamesdphillips, merry xmas 🎄