Merge pull request #292 from mightyiam/issue-291

fix #291

src/modules/eventlisteners.ts

@@ -2,7 +2,7 @@ import {VNode, VNodeData} from '../vnode';
 import {Module} from './module';
 
 export type On = {
-  [N in keyof HTMLElementEventMap]: (ev: HTMLElementEventMap[N]) => void
+  [N in keyof HTMLElementEventMap]?: (ev: HTMLElementEventMap[N]) => void
 } & {
   [event: string]: EventListener
 };