RFC8813 is not recognized as a valid lint source from the CLI

[christopher-henderson]

This is an issue in the CLI frontend. The lint source does indeed run when zlint is invoked with a filter that does not explicitly refer to RFC8813. The following will run RFC8813 successfully.

./zlint -excludeSources CABF_BR github.com

However, the following will simply error out at the CLI

./zlint -excludeSources RFC8813 github.com
FATA[0000] invalid -excludeSources: unknown lint source in list: "RFC8813" 
exit status 1

Issues such as this are found about 1-2 times per year, which is too much. I've made an issue for myself (#878) that this codebase would benefit from integration tests with the frontend.

Thank you @yukoori for submitting the issue!