implement --notify flag (desktop notifications)

[phisch]

When using giph with system keybinds, it is unclear when the recording is completed, and when the encoding process has finished and the final gif is saved.

A flag --notify should enable desktop notifications, which will be displayed by whatever notification server the user has running.

[phisch]

Implemented in b9006b2

Error and success logs now send notifications using notify-send when either -y or --notify is set, where an error sends an urgent, and success a normal notification.