treewide: rename mkTarget's configElements and extraOptions options

Rename mkTarget's 'configElements' argument to 'config' and
'extraOptions' to 'options' to provide a more transparent interface with
the underlying Nixpkgs module system.

(cherry picked from commit 16df6b8448)

doc/src/modules.md

@@ -52,7 +52,7 @@ mkTarget {
   name = "«name»";
   humanName = "«human readable name»";
 
-  configElements =
+  config =
     { colors }:
     {
       programs.«name».theme.background = colors.base00;