support utf-8 encoded text file and sort chapters

Hi !
I forked you rep bookmarkdown, but I found some little problems in it.

1: it only support ascii text, so I did some change so that it can support utf8 encoded text file.
2: when it parse chapters of a book, os.listdir call donot always sort well, so I sort again.

I did some change to fix the two problems and push them to my forked rep, and I think it's necessary to pull request to you.