Add support for "cxxcode" typemap

This allows injecting arbitrary code into the generated C++ classes and
can be useful for defining extra constructors or operators, for example.

Doc/Manual/C.html

@@ -496,6 +496,11 @@ area: 7.068583
       <td>Defines how to transform <tt>ctype</tt> value returned by a function to <tt>cxxouttype</tt>
       </td>
   </tr>
+  <tr>
+      <td><tt>cxxcode</tt></td>
+      <td>May contain arbitrary code that will be injected in the declaration of the C++ wrapper class corresponding to the given type. Ignored for non-class types.
+      </td>
+  </tr>
 </table>
 
 <H3>C Typemaps, a Code Generation Walkthrough</H3>