Improvements for URLs to treat as internal domains

[choldgraf]

Two things we'd like to enhance for internal_domains:

Protocol-relative URLs

One test case to consider is protocol-relative URLs (starting with //).

Domains with a . in the domain name

Multiple-level domains won't get caught:

isExternalUrl("https://mystmd.org.github.io/", "mystmd.org")

returns false.

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• ht @stefanv