Adding ability to define custom screen sizes

added itemsCustom where you can set various dimensions of the browser
width as example:

itemsCustom : [[0, 2], [450, 4], [600, 7], [700, 9], [1000, 10], [1200, 12], [1400, 13], [1600, 15]],

I've updated the code, the documentation page as well as the custom demo. The changes are in separate branch so it can be easily installed and evaluated.

Feel free to optimize it further

Regards