Create skeleton for JupyterLab SpanArray/DataFrame display widget

[frreiss]

This work item covers adding the bare bones of a Text Extensions for Pandas JupyterLab widget. After this work is completed, we will be able to flesh out the skeleton code by adding JavaScript and HTML to do visualization and eventually interactive editing of the displayed data.

Key items to include in this first skeleton:

• Create a subdirectory of our main repository where the JuypterLab widget code will live
• Set up the build process for the widget code
• Extend the CI configuration in https://github.com/CODAIT/text-extensions-for-pandas/blob/master/.travis.yml to also build the widget
• Create a very basic widget that intercepts JupyterLab events to display DataFrames, Series, and SpanArray objects and surfaces a "hello world" HTML message in their place.
• Also have the "hello world" HTML code fetch some server-side data from the widget's backend.