* pick and omit don't treat an empty selection correctly (IMHO). specifically:
  * pick(d) should probably return nothing
  * pick(d, prefix=...) should return only those prefixes
  * omit(d) should return d with all keys copied
  * omit(d, prefix=...) should return d with all keys that have the specified
    prefix removed...

* perhaps use `vars()` during aadict() construction so that it can
  handle argparse.Namespace objects ? ...
