insert-license: new --use-current-year optional flag

Handling years flexibly

You can add --use-current-year to change how the hook treats years in the
headers:

• When inserting a header, the current year will always be inserted
  regardless of the year listed in the license file.
• When modifying a file that already has a header, the hook will ensure the
  current year is listed in the header by using a range. For instance,
  2015 or 2015-2018 would get updated to 2015-2023 in the year 2023.
• When removing headers, the licenses will be removed regardless of the
  years they contain -- as if they used the year currently present in the
  license file.