Customizable copyright string check

[mbeacom]

The goal of this PR is to allow users to provide a custom copyright string. The copyright string can be used in conjunction with a custom template to ensure duplicate copyright headers are not created when the copyright string doesn't match the pre-defined collection: copyright, mozilla public, or spdx-license-identifier.

EDIT: Any guidance here is appreciated, if someone can spare it. Is there anything else I should plan to do for this PR? @willnorris

Fixes #56