Set attributes that provide runtime environment hints

[mojavelinux]

We've been using the convention of setting implicit attributes that identify the runtime environment to allow other settings, such as imagesdir, to be setup properly.

I recommend setting the following implicit attributes for gist.asciidoctor.org:

• env=docgist
• env-docgist
• platform=opal
• platform-opal

This might be used to setup the imagesdir as follows:

:imagesdir: images
ifdef::env-docgist[]
:imagesdir: https://github.com/asciidoctor/asciidoctor/raw/master/images
endif::[]

In contrast, when the document is converted on the GitHub servers, the attributes env=github and env-github are set.