Enables right-to-left books for different languages

Adding { direction: "rtl" } to bookConfig turns it into a right-to-left book. You start on the cover, flip left, and continue going left until you reach the end of the book.

* Pages are added in chronological order [page1, page2]. The array is reversed before pages are added to the DOM.
* Does not affect existing books without a direction config
* Adds example_rtl book for testing (you can move or remove this, if you want)

HTML5-Storybook is an awesome library! :smile:  I'm using its open source interface for an upcoming children's book editor and viewer. We support multiple alphabets, including Arabic, so it's essential that books go in both directions. In the spirit of open source I wanted to contribute code back into the system.