Tweak the rendering of the bibliography to an unordered list.

[odrotbohm]

DISCLAIMER: this is not a serious suggestion as in a proposal to merge. I just needed to solve #76 for me quickly. I'm sure there's a much better way to solve this, but I am completely without any Ruby skills whatsoever.

Previously, the bibliography entries have been rendered as individual paragraphs. This commit tweaks the rendering to rather use an unstyled, left-aligned, unordered list.

This essentially fixes #76 and relates to asciidoctor/asciidoctor-pdf#1889.

[mojavelinux]

This is generally the right idea, though the official solution should work by building a list with the bibliography style and putting that into the AST.

As I mention in the linked issue, it's very important that an extension which produces a bibliography list build the correct semantic structure. That allows converters like Asciidoctor PDF to recognize the element and format it appropriately. Otherwise, the converter just sees it as paragraph text and aren't able to do anything special with it. So turning it in a list is a start, though it should at least have the [bibliography] style on it.