Add support for asciidoc manuals

[adrianovieira]

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(If your issue is neither a both bug nor a request, please write in a free style)

• https://esdoc.org/manual/feature.html
• https://esdoc.org/manual/config.html
• https://esdoc.org/manual/tags.html
• https://esdoc.org/manual/api.html
• https://esdoc.org/manual/faq.html

Add support to render/parse manual in asciidoc format.

Your environment

• OS type and version: Fedora-30
• Node.js version: 12.13
• ESDoc version: 1.1.0