Extender 'ignoreInValidationGroup' to guard observables from being added to validation groups

We show all errors in a list using ko.group's ```errors``` method. We have some observableArrays in the view model for which we created computeds filtering them. That caused validation messages to appear twice. The reason was that group's traverse (when using deep) follows each ```ko.observable``` and ```ko.computed``` inherits from ```observable```.

Therefore I have added an knockout extender 'ignoreInValidationGroup'.
This extender can be used to flag a Knockout Observable to be ignored in validation groups.

When the extender is set the observable will not be added to validation groups.

Rules added to a flagged obserable still apply but the errors are not shown in any group.