0.2.0

• Rule description is now your RuleOp's docstring
  if description is not explicitly provided.
• Supporting virtual plugins: plugins provided by Python
  dictionaries with rules defined by the RuleOp classes.
• Added more configuration examples in the examples folder.
• All xcube rules now have references into the
  xcube dataset specification.
• Introduced mixin classes ValueConstructible and
  derived MappingConstructible which greatly simplify
  flexible instantiation of XRLint's configuration objects
  and their children from Python and JSON/YAML values.
• Made all docstrings comply to google-style.