Prettyify the AST before printing

[mattkrick]

graphql-js print doesn't dedupe fields. this is because it doesn't really matter because the receiving server is going to handle it just fine.

for our use case, it makes sense to deduplicate things because things can be repeated many, many times over.
see graphql/graphql-js#2342

for example, check out this:

    issue(number: 5821) {
      __typename
      ... on Issue {
        __typename
        title
        number
      }
      id
      __typename
      id
      __typename
      id
      __typename
      ... on Issue {
        __typename
        repository {
          nameWithOwner
          id
        }
      }
      id
      __typename
      id
      __typename
      ... on Issue {
        __typename
        ...PokerEstimateHeaderCardGitHub_issue
      }
      id
    }
  }

look at them dupes!
i think these are generally caused by relay, but need to confirm.