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.

Signed-off-by: Parker Moore <parkrmoore@gmail.com>
Update history to reflect merge of #2110. [ci skip]

Closes #2110.