Path Validation
---------------

Some plugins specify a path within the source data. A path is simply an array
of keys to be traversed within the source tree.  The following options are
added by this trait:

**path**
  The array of keys used to specify the path in the source data

**current_value**
  The optional current value in the source path. Used to
  ensure that the specified value exists in the source before manipulating it.
  This must be specified to enable path validation.

**value_path**
  An optional path to be used instead of the normal ``path`` for
  validating the ``current_value``. Used when changing the value in ``path`` but
  testing ``current_value`` in a different ``value_path``.