Divide current layouts into includes so they can be reused by external layouts

[jdesiloniz]

The current layouts built through scala-tags should be divided into partial layouts (aka "includes"). These should be generated and used in the actual layouts (through the {% include %} directive in Jekyll).

Also, all configuration variables (i.e.: site name, repo url, etc...) should be added to the generated config.yml as custom variables, so users that incorporate external layouts have access to them.