Create sample colors and allow the user to change the number of means

You can now specify `K` to the wp script and also several commands at once.

A file named `$WP_FILE.sample.png` is now created in `~/.wallpapers/` and it contains a sample of the colors extracted

So for this wallpaper : 

![Tooltip](https://raw.githubusercontent.com/Mawu3n4/Dotfiles-freezing-dangerzone/master/slim/binary/background.jpg)

You will get this `sample.png` (with k=4) : 

![Tooltip](http://i.imgur.com/VrZJzFS.png)