Issue Description:

When encoding/decoding very big objects or arrays, it can take some time. In clients (UI or CLI apps) it would be reasonable to show a progress indicator for long running tasks like that.
Since JSON(De|En)coder and PropertyList(De|En)coder are both in Foundation, this API could make use of the Progress API in Foundation. But it could also just be a closure which is called with the progress in percent.
I'm not sure how easy it is to calculate the actual progress though as JSON(De|En)coder are using JSONSerialization under the hood which is also not providing an API for progress updates. In theory it could be calculated by dividing the number of processed bytes by the (known) number of total bytes.