FR: Add configuration item for default path to text files

[erezsh]

Right now you can configure each file separately, but more often than not, these files are going to sit in the same path (and not always current directory), using the default names (done.txt, todo,txt). I think adding this option will cover the common use-case better than the current filename and archive_filename, and it can live in addition to them.

I also don't mind writing it myself and sending a PR.