[feature] Notify user upon completion

[yxdunc]

Requested feature
LiPl lets you monitor the progress of a given command toward a --target given by the user. A nice addition to would be to notify the user once --target is reached.

Proposed solution
The solution would be to show a desktop notification once the command reaches the --target value. This notification could be shown only when the user asks for it with another flag: --notify.

Alternatives solutions
Another solution or complementary one would be to let the user know via mail if the command is running on a remote server.

Additional context
Potential rust crate for desktop notifications:

• https://github.com/hoodie/notify-rust