Multitask
Following our discussion in #217, I've implemented a first working version of `MultiDataset`.

There's a function `build_multitask()` which takes either individual `nlp.Dataset`s or `dicts` of splits and constructs `MultiDataset`(s). I've added a notebook with example usage.

I've implemented many of the `nlp.Dataset` methods (cache_files, columns, nbytes, num_columns, num_rows, column_names, schema, shape). Some of the other methods are complicated as they change the number of examples. These raise `NotImplementedError`s at the moment.

This will need some tests which I haven't written yet.

There's definitely room for improvements but I think the general approach is sound. 