index.html: Display post summaries below date

[devurandom]

This adds a little bit of information to the post title by displaying its description or summary, if available.

[Keats]

Can that be a theme settings? The original hyde theme doesn't have summaries so that would be a deviation from it that should be gated.

[devurandom]

Theme settings: Would be fine with me. I can also simply create a derived theme and just leave Hyde alone in its current form.