on build task, copy the CNAME file if it exists

Hi,

When we use middleman-gh-pages and a specific CNAME, we need to copy manually the CNAME file from the source to the build directory.

So I suggest, at the end of the build task, to copy the file to the build folder if a CNAME file exists.

What do you think about that?

Before, we have used submodules directly but the process was a bit longer. Thanks for your gem, it's really helpful!

Kevin