downloaders: make the system more modular
wallpaper-roulette: use cron, not sleep

Usage:
- Start the daemon: wallpaper-roulette <command>
- Change the command and fetch a new image: wallpaper-roulette <command>
- Fetch the next image: wallpaper-roulette
wallpaper-roulette: add Google Images, for the brave
wallpaper-roulette: add a brute force crawler