Add option for colophon including last generated time - default to false

Introduce a colophon option which adds a link to your repo in the footer of the generated report as well as a timestamp.

I found this addition useful in a project at work, allowing the rest of the team to see how the TODO list was generated (and crediting your grunt plugin) as well as leaving a timestamp on the document. 

If you think it's a good idea, leave it in! :+1: