"alterHeightPosition" parameter added

[AxelBriche]

I have added a simple "alterHeightPosition" parameter for control with precision the default position of the background image.

Exemple without parallax("50%", 0.8): http://puu.sh/ciEgw/f585fa5b17.jpg (white margin bellow)

Exemple with my modification and parallax("50%", 0.8, 100); The third parameter add +100px to the top of the background. http://puu.sh/ciEv9/186ea60e53.jpg (add 100px to top, fix the white margin bellow)

We can use this new option for controle the position with precision and adjust the render :)