In order to handle static files in the content generation pipeline while
still being able to easily validate that no output overwrites any other
output without adding special cases we introduce a level above Content
that contains all Output.
The idea is that all output files derive from Output so that they can be
checked against the by_uri map. All files will then follow the discover
-> write loop.
Allow the user to set a summary and make Article extract its own summary
when the Article.render is called and the summary is still missing.
Extracted summaries only work when the first item of a markdown file is
a paragraph.
omicron