Added and employed a method for optionally retrieving values at paths.
Added and employed a method for optionally retrieving values at paths.

The new method will return Optional<Value>.None when a JSON value is found at the specified key path, but that JSON value is .Null.  Employed new method in optionalAtPath(_:_:ifNotFound:ifNull:getter:) in the case that ifNull is true.  As a result, when ifNull is true, callers will get back nil from optionalAtPath(_:_:ifNotFound:ifNull:getter:) if a JSON value of .None is found at that path.

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