Enable creation of multiple docs through GraphQL

[islamaliev]

At the moment gql only supports creation of a single document at a time.

mutation {
  create_User(input: {name: "John"}) {
    _docID
  }
}

It should be possible to create multiple document, like this:

mutation {
  create_User(inputs: [{name: "John"}, {name: "Fred"}]) {
    _docID
  }
}

Note: creation of multiple documents is already supported through CLI.