Provide a link to data source and data package in footer

[Stephen-Gates]

As a data reuser, I what to optionally provide a link to the data source and/or data package, so that viewers can gain a level of trust around the data

Display the link at the bottom of the table.

Inspired by: http://preview.data.gov.au/data_cards/01.html

[rufuspollock]

@Stephen-Gates this is an excellent point. However, i think that would probably be the responsibility of the app using this library rather than being included. I want to keep this pretty focused on rendering the just the graph or table.

Happy to hear your thoughts.