Add an optional 'customHeaders' field to AWSAppSyncClientOptions to easily add headers to requests#469

babouinette wants to merge 2 commits intoawslabs:masterawslabs/aws-mobile-appsync-sdk-js:masterfrom

babouinette:allow-custom-headersbabouinette/aws-mobile-appsync-sdk-js:allow-custom-headersCopy head branch name to clipboard