Feature

- Add the configuration variable 'print.empty.columns'.
- If this variable is set to 'no', columns with all empty values are not
  printed. This variable defaults to 'yes' (thus nothing is changed
  by default).
- Updated taskrc.5.in to mention the new variable.
- Added unit tests.
Documentation

- Updated NEWS file to include new config setting.
- Updated 'show' command so it knows about the new setting.