Full support for nested lists

Ended up rewriting the way lists are handled in order to better support nested lists, both numbered and unnumbered. A few things to note:

* Rather than converting to markdown then manipulating, modify the HTML before passing to reverse markdown
* We now fully semanticize the ol/ul and li relationship, including nesting. Valid HTML FTW
* Added nokogiri styles, to parse the style attributes to detect indentation level, both via `margin-left` and as `mso-list: levelN`
* Switched to html-pipeline for previewing as it has better nested list support

Once this is merged, can go back and DRY things up a bit.

Fixes #10, /cc @daveloyall, @gjtorikian as a better fix to #5.

Potentially resolves #12.