Add is_notice flag to the --classify option

I would like to have an `is_notice` flag to the --classify option. It is useful to more easily collect these to keep their text as a package notice text (such as with the Apache 2.0 NOTICEs)

This can be based on:
- a key, legal file with the word "notice" in their lowered file name
- a key, legal file which is mostly made of license text and mostly matched to a license notice