Fix one-line require for webpack

Webpack fails to require markdown.js

```
WARNING in ./~/codemirror/mode/markdown/markdown.js
Critical dependencies:
6:8-15 require function is used in a way, in which dependencies cannot be statically extracted
 @ ./~/codemirror/mode/markdown/markdown.js 6:8-15

WARNING in ./~/codemirror/mode/markdown/index.html
Module parse failed: /Users/mizchi/proj/codemirror-kitchen/node_modules/codemirror/mode/markdown/index.html Line 1: Unexpected 
token <
You may need an appropriate loader to handle this file type.
| <!doctype html>
| 
| <title>CodeMirror: Markdown mode</title>
 @ ./~/codemirror/mode/markdown ^\.\/.*$
```

It is perhaps bug of webpack but anyway it works.