allow push_back() and pop_back() calls on json_pointer

Putting pop_back() to public and creating a trivial push_back()
method allows users of nlohmann::json_pointer to manipulate an
existing json-pointer by adding or removing keys at the end.

This is useful for traversing a JSON-instance and keeping track
of its "absolute path" at any moment.

In my case for a schema-validator error-handler.

include/nlohmann/detail/json_pointer.hpp

@@ -97,7 +97,6 @@ class json_pointer
         return res;
     }
 
-  private:
     /*!
     @brief remove and return last reference pointer
     @throw out_of_range.405 if JSON pointer has no parent
@@ -114,6 +113,16 @@ class json_pointer
         return last;
     }
 
+    /*!
+    @brief remove and return last reference pointer
+    @throw out_of_range.405 if JSON pointer has no parent
+    */
+    void push_back(const std::string& tok)
+    {
+        reference_tokens.push_back(tok);
+    }
+
+  private:
     /// return whether pointer points to the root document
     bool is_root() const noexcept
     {