Add forceHeight: 'content' option for mobile

I kept having issues on mobile browsers in situations where I had keyframes that didn't 'end' until after the end of the content. When using forceHeight, the height is expanded to allow all keyframes to be
reachable by extending the page content beyond where it would normally end. This small change adds a 'content' option for forceHeight which measures the elements within the skrollr-body and adjusts the maximum scroll area to be within the boundaries of the page content, regardless of the final keyframe(s).

I'm not sure if this is the proper way to address this issue, but it seems like other people have been having similar issues with extraneous content at the end of their pages.

Thanks!