Support for Ragged/Jagged arrays

On this [line](https://github.com/domluna/memn2n/blob/8a4915ac18e95db3828f3ad862609718c28a17e8/memn2n/memn2n.py#L76), it is mentioned there is not support for jagged arrays, but the new Tensorflow v2.1.0 has introduced [RaggedTensor](https://www.tensorflow.org/api_docs/python/tf/RaggedTensor).
It would be nice if support for this feature can be provided in the current codebase.