core: Add 'filepath_debug' option to show included templates

Fixes #572

When mod_development.filepath_debug = true in the admin (checkbox on
development page), pages will have added HTML comments like this:

```
<!-- START modules/mod_admin/templates/admin_base.tpl -->
  ...
<!-- END modules/mod_admin/templates/admin_base.tpl -->
```

This really eases template development, as the developer can now
exactly see which templates get include. Does both runtime and
compile-time includes.

Possible extensions:

- only show the comments to an admin user (as they might break the
  site's layout in some cases)

- Change the comment types depending on the content type (for
  generating non-HTML types)