Attributes description

**For review ONLY – do not merge**

This PR is implementation of https://github.com/apiaryio/api-blueprint/pull/135 in the API Blueprint AST

**TODO:**

- [ ] Better examples
- [ ] Source Maps

> As said above, we only want to represent what the users wrote

Which we will. There is not mis-interpretation when we will fill-in the name so others can directly use it.