Cleaning up the generated TOC

[Irvel]

Hi,

I was able to get a cleaner output of the TOC by using the merged .md files generated by the nostarch.sh script in the book repo:

And by modifiying a bit the .book config file

As mentioned by #3, the img/ prefix in ch20-00-final-project-a-web-server.md needs to be removed so that the build doesn't fail.

This is the output of applying these changes with the 2017-10-28 7db393d version of the book:

PDF
EPUB
HTML