add scroll tracking option

[bguiz]

Currently only pings google analytics when page is first loaded, or we switch to another page.

This adds the ability to track whether the reader has actually read each page (determined by scroll positions and times".

To use, simply add "scroll" to pluginsConfig.ga in book.json.

[bguiz]

Sample usage: https://github.com/bguiz/angular-ember-compare/commit/6d743810cac64c9e0e7d590cb028e6ff50eded78