Do not merge overlapping selection when entering append mode

Fixes #163

src/input_handler.cc

@@ -842,6 +842,7 @@ private:
              kak_assert(false); // invalid for interactive insert
              break;
         }
+        if (mode != InsertMode::Append)
             selections.sort_and_merge_overlapping();
         selections.check_invariant();
         buffer.check_invariant();