Add support for browsing empty directories

Turns out that being able to open an empty directory actually has its
uses:
- If you delete the last file in a directory, you now won't be directed
  to the parent directory anymore. This allows keeping 'd' pressed
  without worrying that you'll delete stuff outside of the current dir.
  (This is the primary motivation for doing this)
- You can now scan and later refresh an empty directory, as suggested by
  #2 in http://dev.yorhel.nl/ncdu/bug/15