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