Add support for es-5 property with underliing observable.

It is very convenient to use es5 properties with getter and setter set to observable. This way I can use property without braces and still subscribe for changes (directly to underlining observable). Knockout knows how to bind such properties - it has `ko.expressionRewriting.twoWayBindings`.
Is there a possiblity for knockout-validation to support such properties?

The issue names no files or tests; start by reviewing how Knockout-Validation integrates with Knockout observables and the referenced ko.expressionRewriting.twoWayBindings behavior. Define done as validation supporting ES5 getter/setter properties backed by an underlying observable, including subscription and binding behavior.

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