Auto populate new task with "what: " (thanks @philipdexter)

Useful feature. Plus, it fixes a bug: Two successive calls to
new_filename() *COULD* return the same filename. It never happened to me
because it's a very slim chance but it is possible.

Closes #4
Show "<NO SUBJECT>" instead of an empty string

This makes it easy to clean up items:

  $ gitodo d :/NO\ SUBJECT
Add housekeeping via "clean_empty"
Always use File.open() instead of File.new()