add loop: boolean to GrainPlayer (#12)

Author: JackCA

index.d.ts

@@ -5328,6 +5328,11 @@ declare namespace Tone {
      */
     grainSize: Encoding.Time;
 
+    /**
+     * If the buffer should loop once it’s over
+     */
+    loop: boolean;
+
     /**
      * If loop is true, the loop will end at this position
      */