Create CNAME file if no exists

[raphaelcohn]

For GitHub Pages, create a CNAME file if none exists, potentially by parsing a host name from configure_getValue swaddle host_base_url.
To be really useful, should also overwrite an existing CNAME file.

However, it is possible that configure_getValue swaddle host_base_url is (a) different for apt and yum and (b) may actually be the original (github.io) name, creating a potential problem. (a) is probably ignorable, as this is very unlikely for users using GitHub Pages.

Is there value in another configuration setting, so at least CNAME is updated on each release? Or does it add to complexity (and a hidden source of bugs)?