Article summaries

This branch introduces a three tiered approach to delivering article excerpts for listings pages and allows basic html markup to pass through into the excerpt.

• If a hand crafted excerpt is written, use that...
• Else, if the article is divided with a <!--more--> tag, use the content above more,
• Else, truncate the article content after so many words (for now, arbitrarily 105).

This custom excerpt code also adds a `more-link` for articles that have no content beyond the excerpt. It is distinctly classed so that it can be hidden with CSS if desired.