Support external commands

[carboniris]

Some users such as myself simply aren't running any of the supported terminals or desktop environments. Rather than opening an issue to add support for something else, we could future-proof the project by adding an exec option. For example:

pokemon pikachu --exec "feh --bg-scale %s"

This way the user can use any command on the image file.

[carboniris]

On the other hand, it could also be useful to format the string with several properties. For example:

pokemon slideshow --exec "feh --bg-scale %filename; notify-send %name"

This would start a slideshow which sets the background using feh and sends the name of the Pokemon as a notification each time it changes.

[samuelhnrq]

I mean it is a great feature, but I think it would be better to implement feh support first as a 'desktop wallpaper' provider first... But then to do that, it would be better to implement the adapter pattern to the supported desktops...