add fmt method for BacktraceFmt

Author: chenyukang

src/print.rs

@@ -95,6 +95,14 @@ impl<'a, 'b> BacktraceFmt<'a, 'b> {
     pub fn message(&mut self, msg: &str) -> fmt::Result {
         self.fmt.write_str(msg)
     }
+
+    /// Return the inner formatter.
+    ///
+    /// This is used for writing custom information between frames with `write!` and `writeln!`,
+    /// and won't increment the `frame_index` unlike the `frame` method.
+    pub fn fmt(&mut self) -> &mut fmt::Formatter<'b> {
+        self.fmt
+    }
 }
 
 /// A formatter for just one frame of a backtrace.