enhancements for PathPrune

- error by default when the time property cannot be found. Can be
  disabled via the boolean 'ignoreMissingTimeProp'.
- support for parsing the name of a node as a DateTime formatted
  string rather than using a DataTime prop. This is used when
  'nameFormat' (a DateTimeFormat string) is not null.
- added an alias for PathPrune called `prune-ymd` with a default
  nameFormat of 'YYMMDD' and `_primary = ttl`. It is used like:
  `prune-ymd: 30 days`.
- added a bunch of tests for PathPrune, although they mostly just
  use nameFormat so that there are less things to mock.