Add option to exclude non referenced manual packages

[dbeusink]

First of all I want to thank you for this great tool! The tool has been really convenient for use in CI/CD pipelines to allow only specific licenses and to report the list of used packages in markdown format.

In the current implementation all non-referenced packages that exist in the manual package information are included in the list of detected packages. I'm using this tool in CI/DI pipelines with a shared manual.json. In our pipelines we use the manual json to fix packages for which no license info could be retrieved automatically. This works great but also adds all the manual packages to the list of detected packages; even when the packages are not referenced at all. In my PR I added an option to exclude non referenced manual packages.

Let me know what you think.
Thanks in advance.

[dbeusink]

Staled. We decided to fork the whole repository and maintain it ourselves now.