Force single app instance on windows

fixes issue #236

src/main.js

@@ -128,6 +128,16 @@ const trayImages = function() {
 }();
 var willAppQuit = false;
 
+// If there is already an instance, activate the window in the existing instace and quit this one
+if(app.makeSingleInstance((commandLine, workingDirectory) => {
+  // Someone tried to run a second instance, we should focus our window.
+  if (mainWindow) {
+    mainWindow.show()
+  }
+})) {
+  app.quit()
+}
+
 function shouldShowTrayIcon() {
   if (process.platform === 'win32') {
     return true;