Adds support for inline callout in a fairly hacky way. These look like:

```
----
POST <1> /_search/scroll <2>
----
<1> words
<2> other words
```

They are supported by asciidoc and the elasticsearch reference uses a
few of them and they look *great*. Asciidoctor doesn't support them and
doesn't have a nice extension point to add them so we hack asciidoctor
fairly shamelessly to get them in.

This cuts the number of failures when building the Elasticsearch
reference down from "then entire screen" to 3.