Add Icon Support to Linux

[CaderIdris]

Add Icon Support to Linux

Description

Notify-send allows for an --icon argument that displays an icon with the notification, this pull request allows the user to specify an icon to use for a successful command as well as a failed one with AUTO_NOTIFY_ICON_SUCCESS and AUTO_NOTIFY_ICON_FAILURE respectively.

Limitations

I don't have a Mac to test with so this feature only works with Linux so far

Changes

• Added logic to _auto_notify_message to add icon arg
• Added test to check if icon arg added after successful command
• Modified test to pass if icon arg not added as blank space is now present at end of output
• Added section to docs explaining configuration for success and failure icons

[CaderIdris]

Adds similar behaviour to the one expected but not current available in issue #54