ENH: Allow configuring null data files and then don't create them.

Request from exatto (http://article.gmane.org/gmane.comp.diary.todotxt/4699), who doesn't want to use done.txt and report.txt.

Also consider character devices (-c; /dev/null is one) in the file existence checks so that no empty file is then created.
Simplify empty data file creation.

No need to use the external cp command; shell redirection will create an
empty file for us.
Merge pull request #116 from inkarkat/enh-allow-null-files

Allow configuring null data files and then don't create them.