Permit YAML blocks to end with three dots

The YAML spec permits blocks to end with three dots (...) in addition to
three dashes (---): http://www.yaml.org/spec/1.2/spec.html#id2760395. Some
programs that work with Jekyll (e.g., Pandoc) prefer the dots to dashes. This
commit permits the YAML metadata block to end with either dots or dashes. It
includes tests.