Now we have the ability to count the number of advisories per repo, we
can create a handy `--summary` view for advisory reports as a way to
provide an overview of the advisories present in each repo, if any.

This also produces a helper method,
`SummariseCountPackageAdvisoriesLike`, to allow us to take advantage of
this logic in the future, until we have completed #231.

Closes #230.