Add script.

usb/set-led-pwm.js

@@ -0,0 +1,14 @@
+#!/usr/bin/env node
+let uhk = require('./uhk');
+
+let programName = process.argv[1];
+let brightnessPercent = process.argv[2] || '';
+
+if (brightnessPercent.length === 0) {
+    console.log('Usage: ${programName} [LED pwm percent]');
+    process.exit(1);
+}
+
+uhk.sendUsbPackets([
+    new Buffer([uhk.usbCommands.setLedPwm, +brightnessPercent])
+]);