Add better editing of array and objects in meta_value

[pippinsplugins]

Right now we only display the serialized data. We should add some form of repeatable fields UI to handle serialized data.

[kevinwhoffman]

A GUI with repeatable fields would be helpful. As an intermediate step, what do you think about deserializing data in the existing textarea and then serializing it again after clicking Update? This would allow developers to edit serialized data in a legible manner.