ensure dir exists first.

Otherwise the first config will fails:

```bash
$ douban.fm config
prompt: Douban Email:  fengmk2@gmail.com
prompt: Douban Password:  
{ [Error: ENOENT, open '/Users/mk2/douban.fm/.configs.json']
  errno: 34,
  code: 'ENOENT',
  path: '/Users/mk2/douban.fm/.configs.json' }
```