This patch makes minor changes to make the code a little easier to
understand. For example, using the `?` operator in some fns that return
an `Option` type.
Add a new `empty`/`e` type to search for empty files and/or directories.
To search for both empty files and directories, use one of the
following:
fd --type empty
fd -te
fd --type empty --type file --type directory
To search for empty files, use
fd --type empty --type file
fd -te -tf
To search for empty directories, use
fd --type empty --type directory
fd -te -td
closes#273
