add key-in helper

[AmilKey]

@jmurphyau ^^

[jmurphyau]

Does this work if the object that contains the keys changes?

That was the worry I had with doing helpers on arrays and objects..

I'm not 100% on adding this even if it did work after the objects were updated.. I think I more do like the idea of keeping this library focused on simple a -- b comparisons, rather than a inside b type comparisons.. For those I think maybe an array-truth-helpers and hash-truth-helpers or something might be better..

I'm conscious of the fact that any helper added to this library is then included in every app that uses it.. Since there is no exclusion functionality at the moment there would be no way for a user to not include it in their code..

[AmilKey]

@jmurphyau ok, I agree with you, I think you can close it