string: add advance operator for tokeniser::iterator

string.hpp

@@ -78,6 +78,13 @@ namespace util {
                 return *this;
             }
 
+            iterator operator+ (int count)
+            {
+                auto next = *this;
+                std::advance (next, count);
+                return next;
+            }
+
             const range_type& operator* (void) const
             {
                 return m_range;