Update the default archetype to specify all relevant frontmatter fields.
Updated the bigimg and comments features to handle "unset" or "null"
values in the frontmatter and default appropriately. Also updated the
bigimg sample to explain both the default and "flow" YAML syntax.
Comments toggeled per-page by setting "comments = true" or "comments =
false" in front matter. If not set, will use new default setting defined
in config.toml.
