giving possibility to maintain multiple selection on mouseHouver

Copy of the code of onClick in onMouseHover regarding multipleSelection in order for it not to be set systematically to false.
I would have personnally removed this line as I dont see why mousehover should impact multipleSelection but this way I'm sure anything remains possible.

src/MVTSource.js

@@ -299,7 +299,7 @@ class MVTSource {
     }
 
     onMouseHover(event, callbackFunction, options) {
-        this._multipleSelection = false;
+        this._multipleSelection = (options && options.multipleSelection) || false;
         options = this._getMouseOptions(options, true);
         this._mouseEvent(event, callbackFunction, options);
     }