Add (Content, [Operation]) -> Content option for patchable's reducer (list mode)

So the existing model is mapping single content and operation to content:

```
(Content, Operation) -> Content
```

But this has potential efficiency issues -- e.g. needless over-invoking of JSONPatch.

With the list mode reducer, a list of json patch operations will incur just one JSONPatch invocation:

```
(Content, [Operation]) -> Content 
```