use the queue doctype in the worker by default

allow the user to override the setting

src/index.js

@@ -54,7 +54,10 @@ export default class Esqueue extends events.EventEmitter {
   }
 
   registerWorker(type, workerFn, opts) {
-    const worker = new Worker(this, type, workerFn, opts);
+    const options = Object.assign({
+      doctype: this.settings.doctype,
+    }, opts);
+    const worker = new Worker(this, type, workerFn, options);
     this._workers.push(worker);
     return worker;
   }