Add static property 'accessile' to onoff.Gpio class

types/onoff/index.d.ts

@@ -28,6 +28,8 @@ declare namespace __ONOFF {
             options?: GpioOptions
         );
         
+        static accessible: boolean;
+
         gpio: number;
         gpioPath: string;
         opts: GpioOptions;

types/onoff/onoff-tests.ts

@@ -14,3 +14,5 @@ setTimeout(function() {
     led.writeSync(0);
     led.unexport();
 }, 2000);
+
+var accessible:boolean = onoff.Gpio.accessible;