Add CSP Layouts

It is currently possible to concatenate multiple CSP files into one view by using the `<%view%>` directive. Whenever I have a website with the same header & footer, but different main content, I can add a new CSP file for the main content, then integrate the header & footer with two `<%view%>` statements.

The problem here is that now I have to keep in mind to add both, the header, and the footer, to every page. Another issue is that now, after the header & footer are separated, they do no longer represent a full HTML file.

I would like to see some support for layouts (which would ideally reside in a separate directory). In this case I could say that a specific page _extends_ from a layout (which is a CSP page by itself). The layout page should be an (almost) well formed HTML page that has a placeholder / statement where the concrete page will be substituted.