Articles should be able to specify the passage that will be used (or reused) as the excerpt in shortened listings.  An _inline_ excerpt would reuse an existing passage of the article or note as the excerpt. A _non-inline_ excerpt would be filtered out of the article or note and only used in excerpted views.

A hypothetical `◊excerpt` tag function would be passed through untouched in the doc and then snagged during the caching process in `crystalize.rkt` (similar to `◊note` and `◊title`).

If not supplied, an article or note's “excerpt” form would be the same as its “full” form. Excerpts will only be used when the note or article is too long to display fully in a listing context.