feat(paths): allow specifying the application root

Set the `paths.app_root` variable to allow the user configure the
directory where the main application resides. This allows keeping the
application files separate from the build system.
feat(paths): allow specifying the application root

Set the `paths.app_root` variable to allow the user configure the
directory where the main application resides. This allows keeping the
application files separate from the build system.