A-Frame plugin: introduce events artargetfound and artargetlost

docs/api/plugin-aframe.md

 
 ## Events
 
+The events below are emitted on the scene.
+
 ### arstarted
 
-This event is emitted on the scene when the main loop of the AR scene is set up, just after the AR session starts.
+The main loop of the AR scene has been set up. This takes place just after the AR session starts.
 
 *Since:* 0.4.2. Previously, this event was called *ar-started*.
 
 
 **Details**
 
-* `ar: object`. A reference to the [ar](#ar) system.
+* `ar: object`. A reference to the [ar](#ar) system.
+
+### artargetfound
+
+An image target has been found.
+
+*Since:* 0.4.2
+
+**Details**
+
+* `referenceImage: ReferenceImage`. The [reference image](reference-image.md) that is linked to the target.
+* `ar: object`. A reference to the [ar](#ar) system.
+
+### artargetlost
+
+An image target has been lost.
+
+*Since:* 0.4.2
+
+**Details**
+
+* `referenceImage: ReferenceImage`. The [reference image](reference-image.md) that is linked to the target.
+* `ar: object`. A reference to the [ar](#ar) system.

plugins/aframe-with-encantar.js

             }
         }
 
+        const ar = this.ar;
+        const scene = this.el.sceneEl;
+        tracker.addEventListener('targetfound', ev => scene.emit('artargetfound', { ar, referenceImage: ev.referenceImage }));
+        tracker.addEventListener('targetlost', ev => scene.emit('artargetlost', { ar, referenceImage: ev.referenceImage }));
+
         return tracker.database.add(referenceImages).then(() => tracker);
     },