Reference blocks by their anchor in the layout

[gwennlbh]

Instead of having to rely on positional indices (which may change when adding a new block before the end), naming blocks by their anchors in the layout would provide a much clearer and easy-to-maintain way to specify the layout:

---
layout: 
- [p@intro]
- [p@why, m@problem]
- [p@solution]
- [l@source, l@product]
---

@intro
This project blah blah blah

@problem
![some alt text](./thing.png "caption")

@why
Lorem ipsum dolor sit amet

etc.

This could even be simplified to

layout: 
- [intro]
- [why, problem]
- [solution]
- [source, product]

Of course, the position-based syntax would take precedence over this (so no naming your anchor "m1", what a bummer)

(using the lighter syntax for anchors, see ortfo/db#71)