Respect existing copyright assignment in LicenseHeaderStep

[basil]

The MIT license template contains a copyright assignment line:

Copyright (c) [year] [fullname]

jenkinsci/jenkins is an 18-year-old MIT licensed project, and our files do not all have the same copyright assignment. For example, one file might have

Copyright (c) 2004-2009, Sun Microsystems, Inc., Kohsuke Kawaguchi

and another file might have

Copyright (c) 2015, CloudBees, Inc.

With a template like

Copyright (c) $YEAR [fullname]

Spotless currently changes the above to

Copyright (c) 2004-2009 [fullname]

and

Copyright (c) 2015 [fullname]

We would like it to respect the existing copyright assignment. Reassigning copyright for all files to the same owner is not an option for a project of our age with a complicated lineage dating back to Sun Microsystems, a corporate entity which no longer exists in its original form.

[nedtwigg]

• dupe of Support author name in LicenseHeaderStep #323.