allow map to define custom map events

Add handlers and listeners for events explicitly passed as a prop.
This addresses fawmi/vue-google-maps#47

src/utils/bindEvents.js

@@ -1,6 +1,12 @@
 export default (vueInst, googleMapsInst, events) => {
   for (let eventName of events) {
-    if (vueInst.$gmapOptions.autobindAllEvents || vueInst.$attrs[eventName]) {
+    const propName = `on${eventName.charAt(0).toUpperCase()}${eventName.slice(1)}`.replace(/[-_]+(.)?/g, (_, c) => c ? c.toUpperCase() : '');
+
+    if (vueInst.$props[propName] || vueInst.$attrs[propName]) {
+      googleMapsInst.addListener(eventName, (ev) => {
+        vueInst.$emit(eventName, ev)
+      })
+    } else if (vueInst.$gmapOptions.autobindAllEvents || vueInst.$attrs[eventName]) {
       googleMapsInst.addListener(eventName, (ev) => {
         vueInst.$emit(eventName, ev)
       })