support public_root configuration to help organization static files

The idea is that any file or folder inside of the specific public_root folder will get copied to the root of the generated site. I wasn't sure how files with yaml headers should be treated...I decided to not apply this logic to them. Can't decide which way is more consistent with what a user would expect

Instead of having your site look like:

index.html
resume.pdf
assets
   images
      unicorns.jpg
assets
    css
    main.css

You can put all the static and non-yaml files you want under the public_root => '/public':

index.html
public
   resume.pdf
   assets
      images
         unicorns.jpg
   assets
      css
        main.css