s9y-db importer permalink, tag, excerpt, and semantic HTML improvements

[judebert]

These commits support plugins and properties required to migrate judebert.com from S9Y to Jekyll. The features include:

• support permalinks from the entryproperties plugin
• duplicate S9Y "extended body" functionality with Jekyll excerpts
• import tags and categories
• convert iframes and Serendipity image/captions to semantic HTML figures
• use relative links instead of absolute
• entryincludes plugin

Documentation has been updated to describe the new functionality.

[judebert]

All PR comments addressed. script/cibuild passes on my computer.

[ashmaroli]

I still see the following in the diff:

if options[:clean_entities]
  options[:clean_entities] = require_if_available("htmlentities", "clean_entities") if options[:clean_entities]
end

if options[:markdown]
  options[:markdown] = require_if_available("reverse_markdown", "markdown") if options[:markdown]
end

[judebert]

Friendly ping: I think I pushed another update addressing all issues, about 3 weeks ago. Is there anything else I need to do to merge?

[ashmaroli]

addressing all issues, about 3 weeks ago.

Apologies, @judebert. This had slipped out of my head. Will post a fresh review shortly.

[parkr]

@jekyllbot: merge +minor