Enable multiple gists

I'd like to handle the following:

- A single gist that shows one book from `read` and `currently-reading` (this is the current implementation)
- A single gist to show the last 3 books from `read` and their rating (out of 5)
- A single gist to show ~3 books from `currently-reading`

We could handle this by just allowing the user to input 3 gist IDs and update them accordingly.