Project: parcel
Task (challenging): Add `language` option for markdown parcel plugin

For example:
```js
import md from './hello.md'
```
If langauge is 'en', `./hello.md` will resolved to `./hello.en.md` if it exists.
If langauge is 'zh', `./hello.md` will resolved to `./hello.zh.md` if it exists.
If both `./hello.en.md` and `./hello.zh.md` are not found, fallback to its original path `./hello.md`

Besides this, set language="zh" as default value.