Add support for local pfxnarc_settings.yml

As a pfxnarc user & repo owner

I want to control / override some functions of pfxnarc

So that I can tailor it's behaviour specific to my project

Acceptance Criteria

1. Can I set a custom pattern that allows me to exclude files (matching the pattern) from the narc scanner?
2. Can I customise the failure severity of the tool so that I can elect the severity at which the tool will prevent CI pipeline progressing?