Added flatpak support

Hi, this commit adds flatpak notification support, if there are any updates on your installed flatpak apps. I've added the flag -p --flatpak to enable the check. By default it's not used. It happens quiet often, that flatpak updates do not receive a new version number as the software underlying the flatpak is not updated by itself. To not confuse people by showing an update from the installed version by the same version (ex. version 1.01 -> 1.01), in case the flatpak does not have a version or the update does not increment the version, the commit id of the flatpak is shown. Info: As the flatpak update command does check for updates online, although the flag --cached is used, this can be energy consuming for devices not attached to the grid, and bandwith consuming, during the "offline" checks.