Add USBDevice.isSuspended()

Based on code originally by Rob van der Veer <rob.c.veer@gmail.com>, this adds
USBDevice.isSuspended(), so user sketches can run custom code in their `loop`
methods after checking if the device is suspended or not.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>

cores/arduino/USBAPI.h

@@ -65,6 +65,8 @@ public:
 	void detach();	// Serial port goes down too...
 	void poll();
 	bool wakeupHost(); // returns false, when wakeup cannot be processed
+
+	bool isSuspended();
 };
 extern USBDevice_ USBDevice;
 

cores/arduino/USBCore.cpp

@@ -855,4 +855,10 @@ bool USBDevice_::wakeupHost()
 	return false;
 }
 
+bool USBDevice_::isSuspended()
+{
+	return (_usbSuspendState & (1 << SUSPI));
+}
+
+
 #endif /* if defined(USBCON) */