Modify AutoUpdater behavior on macOS.

The AutoUpdater requires that macOS builds be code signed. That is currently not possible.
As a workaround, the autoupdater on mac will now alert users that an update is available and give the option to download the file directly from GitHub.
Closes #16.

index.js

@@ -22,6 +22,9 @@ function initAutoUpdater(event, data) {
         autoUpdater.autoInstallOnAppQuit = false
         autoUpdater.updateConfigPath = path.join(__dirname, 'dev-app-update.yml')
     }
+    if(process.platform === 'darwin'){
+        autoUpdater.autoDownload = false
+    }
     autoUpdater.on('update-available', (info) => {
         event.sender.send('autoUpdateNotification', 'update-available', info)
     })