Extended Extension - Now supports Notification Sound

[ChitturiSaiSuman]

This Extension now supports Notification Sound

Note: Beware of Initial Setup. Multiple unread mails can cause the extension to play the Notification sound multiple times in a row.

[shumingch]

Hi. Thanks for the PR. Please make the following changes:

• Remove the python file. I only want JavaScript in this project.
• Remove the sound file. I don't like it and the user should be able to input their own sound file. Maybe you could find a way to play gnome default sounds.
• Add a setting to enable sounds and make the default to false. Most users of this extension probably don't want to suddenly hear a bunch of sounds.

I found a similar extension that plays a beep sound on a timer with a setting for alternate commands, maybe you could draw inspiration from that: https://github.com/rusins/beep-timer