Changes in version 0.5.0

- Add `#:omit-fields` keyword argument to `task->string` and
  `group-tasks-result->string`. Pass a list to limit the fields present
  in a task line. Note that if you specfiy `'creation-date`, you must
  specify `'completion-date`, too.

- Add `--omit-fields`/`-o` option to `todoreport`. Use this to limit the
  task fields in the output. Omitting the creation date ("crd") omits
  the completion date ("cod"), too. Use the special value "auto" to omit
  the simple fields used in the `--group-by` option.