Add Merge Feature

Because of a bug in FOSSA that caused many attributions to be lost, and a long estimate from the on fixing it, we need a mechanism to cope.

This is accomplished here by adding a --merge-with flag to the fossa-with-cache script. This flag will cause the specified file to be merged into the generated notice as part of "cleaning" it. If the existing NOTICE is used, attributions can never be lost from one run to the next, but new attributions are reported as a difference to be committed.