add unused-ignored-advisory config setting

**Is your feature request related to a problem? Please describe.** We use a single `deny.toml` across multiple projects, and have some items in `advisories/ignore` that only apply to individual projects. This emits a warning on every other project of the form `advisory-not-detected`. **Describe the solution you'd like** Add a new configuration option `advisories/unused-ignored-advisory` with the options `warn` (default), `allow`, and `deny`, just like `sources/unused-allow-source` and `licenses/unused-allowed-license` **Describe alternatives you've considered** We could edit the TOML file for each project, I guess, but I'd prefer not to.