Render into confluence storage format with macros

[rquinio]

I've been prototyping the rendering of documents into Confluence storage format, using relevant Confluence macros to have a more "native" look & feel and bypassing some limitations (covers issues #3 and #18). I also just discovered via https://discuss.asciidoctor.org/Confluence-erb-templates-td5184.html that @mfriedenhagen actually tried something similar a while ago (though I used haml not erb)

Since this project seems no longer active and was more focused on the upload to Confluence, I'm leaning toward releasing that as an independent ruby library - which could then be combined with asciidoctor-confluence to switch from xhtml, or used with any other Confluence REST API client lib.

Let me know if you think a different approach is better !