Ensure that domains resolve

[stuartpb]

This should be a step further, taken by testing: if the profile's name is not a resolvable domain name with a ".yaml" extension, a mistake has likely been made (like a misspelling), and this would catch that.

[stuartpb]

I'm not sure how often this really happens, though, except when a site dies: I think it'd be better to write a whole tool at some point that runs through the urls listed in a profile and lists what comes back (HTTP status-wise), as well as a test for each filename's root domain.

The purpose of that tool would basically be to see which profiles are most direly need in review: if a lot of 404s and/or redirects are happening, something's probably changed (though, of course, this isn't guaranteed, as some URLs profile a redirecting path, so it can't be used as a test).

[stuartpb]

This might make sense as an extended test for pull requests adding new profiles (though see #123 for the current roadblocks around that).