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