Add configured license amendment content

[jonabc]

This change allows users to specify file paths to custom license terms provided by a library owner. Additional terms content files paths are specified per dependency type and name, and the files are treated as additional licenses for the dependency. See the added documentation for details on how to use the added additional_terms configuration option.

/cc @mlinksva I'm curious for your thoughts here - is this an appropriate change?

1. It seems like there is some amount of risk in allowing users to specify content that would be treated as legally binding
2. Should the extra terms be treated as a legal notice rather than a license?

UPDATE renamed amendments to "additional terms"

[jonabc]

Ignoring the cabal failures, those tests are having issues during their setup steps. I'm merging this in and will put out a release once the cabal tests are fixed in a separate PR.