Removes code duplication (code climate)

No gain but it's "better": https://codeclimate.com/github/Mogztter/asciidoctor/compare/code-climate
This method isn't specific enough. These positional attributes are only relevant for a media block. There are other places that do something similar that could just as easily be called `get_posattrs`.

What I'd like to do here long-term is to actually have some sort of central place where we deal with all the position mappings...perhaps even in a configuration so that they are not scattered throughout the code. Perhaps we can chase that down in a separate issue.