Optionally hide some articles from the front page

[bobbermac]

After some exposure on the front page, it will be desirable for some articles with lower interest levels to not be displayed there. This change introduces support for this.

Tasks:

• Update post source files with a boolean metadata element (e.g., an rfc5322 header) like "Show-In-Headlines: True"
• Convert headline boolean to idiomatic Clojure upon parse: Show-In-Headlines -> headline?
• If empty/nil set to true
• If false, when building list of headline stories, don't include in the top-X (10, by default)