implemented Buffered method

reader.go

@@ -47,6 +47,10 @@ func (r *Reader) Rune() (rune, error) {
 	return rn, err
 }
 
+func (r *Reader) Buffered() int {
+	return r.src.Buffered()
+}
+
 // PeekRune returns the next rune in r without advancing reader position.
 // The next read will return the same rune again.
 func (r *Reader) PeekRune() (rune, error) {