Consider supporting a domain option that creates a custom CNAME

[cvan]

I've run into conflicts where I had a CNAME checked into the repo, but I wanted to push the project to my fork, and it wouldn't work because you can't have two different repos sharing the same CNAME.

I've seen a couple similar gh-page deploy scripts do something similar. Here's one I found: https://github.com/nfroidure/buildbranch/blob/8d3cfd/src/index.js#L61-L64