Hi,

I found `ox-md` exporter drop src-block language information.

My patch fixes the behavior; to embed src-block language information

using markdown src block grammar.

1. Open some buffer

2. Tnsert below code

3. Turn on `org-mode`

4. `C-c C-e m M` (export as markdown in temp buffer)

## org source

```org

#+begin_src python
print(1 + 2)
#+end_src

#+begin_src emacs-lisp
(print "hello")
#+end_src

#+begin_src
something source code
#+end_src

```

## before

```markdown

# Table of Contents



    print(1 + 2)

    (print "hello")

    something source code

```

## after

```markdown

# Table of Contents



```python
print(1 + 2)
```

```emacs-lisp
(print "hello")
```

```
something source code
```

```