Add "draft" feature

Need a `draft` command that would create a draft of a new post. Basically the post workflow should go like:

```
idea -> draft -> publish
```

We should provide commands for all of these. Of course the idea stage could simply be that—an idea in your head. But it's good to reduce friction in recording ideas since you never know when a good one will come along.

This may be as simple as merging in #1, but I'd also like to experiment a bit with the API to see what feels natural.