Widget-periodictable

This is a new version to support JLab2.1 with some new features:

* Custom colors for noselected elements.
* Make it possible to divide the selected elements into different states with custom colors.
* Custom colors for disabled elements.
* Hover state information for each element.
* Get elements from state.

This version used the updated cookiecutter, which supports both Jupyter notebook and JupyterLab.