Render rainbow indent guides

To make the indent guides more helpful, they could be rendered in the same rainbow colors as their corresponding bracket. And, while at it, only if they have a corresponding bracket.

It would look like so:

<img width="639" alt="image" src="https://user-images.githubusercontent.com/30010/79913688-5223fa00-8424-11ea-91c1-537fe6c728f7.png">
