Fix doi urls in prefix duplication, and pmids/pmcids

[cormacrelf]

Fixes #127, fixes #131

• strip url prefix from prefix="... https://doi.org/" (etc)
  • internal note, simplifies the OutputFormat API to just take a Link and return a Self::Build. The list of url-ish variables didn't need to be specific to each OutputFormat implementation (we do only have two, but yeah)
• Fix PMID / PMCID confusion
• match more variants of PMID/PMCID urls
• Confirm doi prefix-stripping works with link_anchors disabled

One note here is that PubMed has started redirecting PMID urls to e.g. https://pubmed.ncbi.nlm.nih.gov/34706787/, so that might be a better canonical url format. But for the moment it just does what the csl 1.1 spec says.

One more note, even when link anchors are turned off, any of the recognised prefixes are still stripped and the canonical URL prefix, which is always HTTPS, is put in its place.