Ensure order of chapters

First off..  Awesome tool! :beers: It has some great features and I look forward to using it for writing in the future.  I noticed that sometimes the generated table contents does not exactly match that of the file names.  Fixed it by throwing a sort on the file list it is joined with `"\\\n"`.

Many thanks for making this cool ebook generator.

P.S. Also I was wondering if there is a license for the code? Didn't see one when I glanced in the source or on rubygems.