Delete err property if no errors. Arrays of validator fns and more.

* Xtrudr instance now has its `err` property deleted when invoked if
  there are no errors.

* Validator functions now must only return a single string for errors.

* Validator functions may return `undefined` to let the input pass
  through "as is".

* The objects passed to require/permit containing the validator
  functions may now have arrays of validator functions as property
  values.

* The permit/require xtrudr methods may also take arrays of strings
  as an argument.