Various enhancements

[sh-ow]

This pull request includes various different enhancements:

1. Add user_output function to centralize output to user. This adds support for notifications (via option) and option to disable local output.
2. The three outputs still default to LVDS1/HDMI1/VGA1 but can be assigned different names via options. Helpful if laptop screen is eDP1 and external screen DP1 (example for modern laptops). This also adds more flexibility to display configuration by changing the names.
3. Add systemd unit for reconfiguring displays after wakeup from sleep. fixes #8 ?

Hint: you can use cherry picking if you only want to merge some commits. I tried to separate all different changes in different commits.

[findesgh]

Like your enhancements, good stuff!