Test sample data style with yamllint

Add a tox environment and associated support for running the
yamllint YAML style checking utility. This tool should be used to
check all edits made to any included YAML files for ease of
readability and future maintenance. It can also be used to check
YAML data generated by the mudpy engine, as its output style matches
the expectations of yamllint's default ruleset. One override has
been added via the included .yamllint configuration file, to prevent
it complaining about the extremely long PBKDF2 password hashes in
account elements which can't effectively be line-wrapped when
written.