text: Title: Parse files with `.robot.rst` extension automatically
  Body: Robot Framework can parse reStructuredText files having `.rst` or `.rest` extension, but to avoid parsing unrelated reST files that support needs to be enabled separately by using the `--extension` option. To make using reST files more convenient, we could automatically parse files with `.robot.rst` extension.
  
  This is easy to implement, because we already support for such multipart extensions with custom parsers (#1283).