Adds support for CNAMEs to CNAMEs

[dmalan]

Fixes #111.

[parkr]

I dig this! Please introduce a test for this.

[dmalan]

Sure, @parkr, would it be best for GitHub's team to create a new record in github.com (or elsewhere) that's a CNAME for, e.g., mattr-.github.com (which is, itself, a CNAME for github.github.io) so that GitHub fully controls the test (rather than use a third-party domain like our own)? Looks like @benbalter added a test using that particular record?

[parkr]

@dmalan I'd love to use stubs here instead of relying on public DNS. Is that something you'd be willing to give a try? If not, I can try my hand at a test.

[parkr]

I have a test written and passing on a local branch but can't push changes to this PR. I'll merge this and push up a separate PR for that.

[dmalan]

Ah, thank you, @parkr! Apologies for the belated reply, afraid this fell off my radar!