GitHub Pages: Is it possible to map multiple sub-domains to the same repository?

Question

In my urig.github.io GitHub Pages repository I have setup:

1. A simple index.html homepage in the root folder /.
2. A Jekyll generated blog in the /blog folder.

Is it possible to map my custom sub-domain www.urig.io to point to the homepage at / and at the same time map blog.urig.io to the /blog folder? If so, how can I do it?