Fix support for linkattrs

Per https://github.com/elastic/docs/issues/505#issuecomment-459163236, I would like to use the linkattrs functionality described here:

https://asciidoctor.org/docs/asciidoc-writers-guide/#target-window-and-role-attributes-for-links

When I try any of the examples there in the Glossary, for example:

> == Introduction
>
> :linkattrs:
>
> Chat with other Asciidoctor users on the http://discuss.asciidoctor.org/[*mailing list*^, role="green"].

the attributes are not recognized. They're output as:

`mailing list^, role="green"`