Multi Monitor support

[Skunkynator]

Instead of drawing to the entire XScreen allow specifying one or more monitors to draw to. Also optionally allow specifying different shaders for different monitors.

• [x] #9
• [ ] Use the specified arguments to draw to the correct monitors for both root and background mode

[danielfvm]

Add additional arguments to specify the monitor

I think to be able to do that we will need to upgrade the arghandler.c. Especially when specifying different shaders to a specifc monitor.

Use the specified arguments to draw to the correct monitors for both root and background mode

Working on it in development branch. It works already for the background mode, root mode still needs some more work.