Add shottr as supported application

[ridicolos]

Hello everyone,

I would love to see the app shottr supported by mackup. It is a great tool to do and work with screenshots.

I already tried to do this myself but failed. But would love to make an PR with a little bit of help.

This is the file I created mackup/applications/shottr.cfg

[application]
name = Shottr

[configuration_files]
Library/Containers/cc.ffitch.shottr/Data/Library/Preferences/cc.ffitch.shottr.plist

Mackup also syncs this file without a problem. But for some reason, the settings just won't apply inside the application. I heard that this could be related to a caching with the UserDefaults in MacOS, but I don't have any expertise in that field.

Anyone has an idea?