use markdown

Note that you must write like following in a haml file;

```index.haml
...
  %body
    ...
    ~ markdown(:index, layout: false, fenced_code_blocks: true)
    ...
```

If you write `= markdown(...)` instead of `~ markdown(...)`,
the generated HTML file will be

```
...
  <body>
    ...
    <pre><code>cd
    ls
    </code></pre>
    ...
  </body>
...
```

then, the output will be like following;

```
cd
    ls
```

"~" makes your code block in the markdown file to be generated with "&#x000A;".
"&#x000A;" is "\n".

ref.
  http://docs.komagata.org/4250
  http://blog.s21g.com/articles/1462
  http://qiita.com/items/ac3082890bf90a6df0c5