Add dumping for types with to_json free function.

docs/doc/api.rst

@@ -321,6 +321,7 @@ There's an API in place to dump (i.e. serialise) C++ objects into JSON construct
       as an object. The key (i.e. ``p.first``) will be dumped in accordance to :func:`json::key` while the value will be
       recursively called with :func:`json::dump`.
     - If the type is :class:`value` then it will print with the above in mind with its internal value.
+    - If the type has ``to_json`` then it will call it and then dump the resulting value recursively.
 
 .. function:: void key(OStream& out, const T& t, const format_options& options)
 

jsonpp/dump.hpp

@@ -259,6 +259,11 @@ inline OStream& dump(OStream& out, const T& t, format_options opt = {}) {
     return out;
 }
 
+template<typename OStream, typename T, EnableIf<has_to_json<T>, Not<Or<is_object<T>, is_string<T>, is_array<T>>>> = 0>
+inline OStream& dump(OStream& out, const T& t, format_options opt = {}) {
+    return dump(out, to_json(t), opt);
+}
+
 template<typename T>
 inline std::string dump_string(const T& value, format_options opt = {}) {
     std::ostringstream ss;