Webmention for senders and receivers: find the endpoint of a target and
send it a Webmention, take Webmentions at an endpoint of your own, and
verify their sources.

The code follows the W3C Recommendation of 12 January 2017 at
https://www.w3.org/TR/webmention/ and cites its sections, with the
erratum that the editor's draft at https://webmention.net/draft/ fixes.
A receiver is a server with a public URL, since a sender posts to its
endpoint. What you need to show a mention is read from its source by
dk.simongray.webmention.mention.

The steps behind these functions are in dk.simongray.webmention.sender
and dk.simongray.webmention.receiver, after the two roles of the
standard, and aren't part of the API.

What you need to show a mention, read from the microformats of its
source: its kind, its author and its content.

The kind follows the Response Type Algorithm of Post Type Discovery, the
W3C Note of 18 January 2018 at https://www.w3.org/TR/post-type-discovery/.
The author follows the authorship algorithm at
https://indieweb.org/authorship-spec, and the h-card of a page follows
https://microformats.org/wiki/representative-h-card-parsing, as
microformats-clj reads them.

The steps behind these functions are in dk.simongray.webmention.items
and dk.simongray.webmention.content, which aren't part of the API.