Add HTMLAnchorElement#download

Author: faveoled

dom/src/main/scala/org/scalajs/dom/HTMLAnchorElement.scala

@@ -16,6 +16,13 @@ import scala.scalajs.js.annotation._
 @js.native
 @JSGlobal
 abstract class HTMLAnchorElement extends HTMLElement {
+ 
+  /**
+    * A string indicating that the linked resource is intended to be downloaded rather than displayed in the 
+    * browser. The value represent the proposed name of the file. If the name is not a valid filename of the 
+    * underlying OS, browser will adapt it.
+    */
+  var download: String = js.native 
 
   /** Is a DOMString that reflects the rel HTML attribute, specifying the relationship of the target object to the link
     * object.