Make RecyclerConverterAdapter's ViewHolder overrideable

[DSteve595]

This allows subclasses of RecyclerConverterAdapter to also use their own subclasses of RecyclerConverterAdapter.ViewHolder. This enables the use of AdvancedRecyclerView's (https://github.com/h6ah4i/android-advancedrecyclerview) drag-and-drop, which requires that ViewHolders implement a certain interface.

[BenTilbrook]

Might also need to expose RecyclerViewContainer, and perhaps a factory function for creating
the ViewHolders. I'd like to look into this a bit myself later, since I want to carefully manage the API of RecyclerConverterAdapter to avoid exposing unnecessary details. In the mean time, copy the RecyclerConverterAdapter source into your code base and we can discuss any interoperability issues you run into.

See #48

[DSteve595]

Gotcha. That's what I'm currently doing, and so far I haven't needed to extend the container. A ViewHolder factory would solve my issue,