Overhaul link fixing

[zkamvar]

Since I have a much better abstraction for creating arbitrary XML to add into the document, my solution for reference link tags with missing references (aka klink, the links that were imported from a separate file) is a bit outdated and could stand to be defragged.

There are also a couple of issues that I need to tackle:

1. ampersands need to be protected when creating links in xml_new_paragraph() (related to Clean newly created anchor links ropensci/tinkr#51). This is especially problematic when authors forget to escape ampersands.
2. I need to think about how we post-process liquid links from reference-style links. They might be fine, but I need to make sure they can be fixed in post.