preserve newline before EOF

[johnyf]

As observed with git diff, markdown-toclify consumes the newline at the end of a file. It seems that the final newline is ignored when reading the file, and not appended when forming the output.

[rasbt]

Thanks for your feedback @johnyf (regarding this and the other issues you recently opened). I am currently a bit too involved in other projects so that I probably can't get to it immediately, but I will try to address some of these points in future. In the meantime, I'd welcome pull requests if you have some good solutions in mind :)