[PubMed] Add hidden pref to change link behavior

The main point of this was to add a hidden pref so that PubMed links could be stored in the URL field. It's now controlled by the `extensions.zotero.translators.pubmed.linkAsURL` preference.

I also fixed a couple bugs and restructured the code to make it easier to work with.

It's not exactly obvious from the diffs, but I added callNumbers, which for journal articles are PMIDs (it's redundant with the extra field, but I thought it was appropriate), but but books are accession numbers.

Next step is to add recursive behavior.