add a copyright year value to JSON output

In a CI scenario one might want to check if the copyright has the year at which the last commit changed the file. For that Scancode-toolkit could provide extra "copyright year values" in the JSON output (from_year, to_year).

Simple example for a pre-commit hook to check the year:
http://damien.lespiau.name/2013/01/a-git-pre-commit-hook-to-check-year-of.html