Added support for custom fixed header height

Defining a value for `data-scroll-header` overrides calculated `scrollHeader.offsetHeight`. 
Sometimes this is more versatile than depending on `.offsetHeight` which e.g. doesn't include margins. I know the "fixed header option" it's not your main goal for the script but I use it all the time. Switching to data-attributes makes it really elegant to include this feature. So maybe you could include it for everyone to use… Thanks!