Allowing nesting of variables

Today the `asciidoc` system allows nesting of variables e.g:

```
:es: Elasticsearch
:es-sql: {es} SQL

:webhook: Webhook
:webhook-cm: {webhook} - Case Management

:ml: machine learning
:ml-jobs: {ml} jobs

:es-repo: https://github.com/elastic/elasticsearch/
:es-issue: {es-repo}issues/
:es-pull: {es-repo}pull/
:es-commit: {es-repo}commit/
```

While ideally we don't take on this complexity it does seem very pervasively used.

Should we implement this or force a more simple `key : value` on to our writers and developers?

@bmorelli25 @KOTungseth @reakaleek