Allow automated test missingdocs

[KeithWM]

We wish to use the missingdocs functionality in a CI/CD environment. For this, it is useful to have the missingdocs function not only print out missing documentation, but also return the number of missing documents. This can then be verified using a @test missingdocs(...) == 0.

[mortenpi]

LGTM, thanks! Something to note here though is that missingdocs is not actually part of Documenter's API. But it's also unlikely to go anywhere or change it's semantics any time soon.