improvments to yaml deading and writing
 * Conditional document tags in yaml. Variable ACCEPTABLE_YAML_DOC_TAGS can specify additional doc tags that should start with index or define.
        Example:
            ACCEPTABLE_YAML_DOC_TAGS: define_$(TARGET_OS)
            Would later read sections starting with !define_Win - if Win is the target os.
 * fixed bugs in interactive mode list command.