Notification timeout too short

At least on my system (Ubuntu, using `notify-send`), the default timeout of 100 is too short, producing a very brief flash-notification. I assume this is the line to blame:

https://github.com/jaredly/vscode-background-terminal-notifier/blob/9cae55f503fbbd25b55e4d1bbe5e2d10481eb721/index.js#L42

I suppose this behavior is OS-dependent, so I hope you agree it would be nice to let the user customize this.

Start at index.js line 42, where the notify-send timeout is set to 100, and review how notifications are invoked on Ubuntu. Define how a user-configurable timeout should be supplied, then verify that the notification duration can be changed from the default without breaking existing behavior.

Written by the indexing model from the issue text.