config: add unattended-upgrades config, fixes #53
 * Auto upgrade from ttesting, testing-updates, proposed-updates, Debian-security
 * Enable Unattended-Upgrade::MinimalSteps to allow reooting/shtting down during upgrades
 * Autoremove unused packages during upgrades
 * Update package lists automatically
 * Auto download upgreadable packages
 * Autoclean package cache every 7 days