Add 'getOperationName' option

[onionhammer]

Add support for appending an operationName to graphQL operations, usage:

const graphClient = new AwesomeGraphQLClient({
  endpoint: "/graphql",
  formatQuery: (query: DocumentNode) => print(query),
  getOperationName: (query: DocumentNode): string | undefined => {
    for (const node of query.definitions) {
      if (node.kind === Kind.OPERATION_DEFINITION) {
        return node.name ? node.name.value : undefined;
      }
    }
  },
});

resolves #86