Add support for Watson Knowledge Studio document collection format

[frreiss]

Watson Knowledge Studio (https://cloud.ibm.com/docs/knowledge-studio?topic=knowledge-studio-wks_overview_full) can export annotated document collections (also known as "ground truth") as a collection of JSON files. We should add functions under text_extensions_for_pandas.io.watson to import collections in this format into DataFrames.

We'll need to track down some technical documentation on the file format to make this import functionality robust.