Checkmark support

Hi,

I love snarkdown (seriously, it's really handy) and I already used it in a few projects.

But there is one thing missing: Checkmark support

Here is an example:

- [ ] Unchecked
- [x] checked

```markdown
- [ ] Unchecked
- [x] checked
```

It really comes in handy when making to-do lists

Please consider adding it.
Thanks

首先定位 snarkdown 的 Markdown 列表解析入口点，以及现有的列表语法测试。使用此 issue 中未选中和已选中的示例作为预期输入，然后验证两种形式都会生成预期的复选标记列表输出。

由索引模型根据 Issue 内容生成。