Adds `directories` to the set of plugin directories.

        `directories` may be either a single object or a iterable.

        `directories` can be relative paths, but will be converted into
        absolute paths based on the current working directory.

        if `except_blacklisted` is `True` all `directories` in
        `self.blacklisted_directories` will be removed

def add_directories(self, directories, except_blacklisted=True):
        """
        Adds `directories` to the set of plugin directories.

        `directories` may be either a single object or a iterable.

        `directories` can be relative paths, but will be converted into
        absolute paths based on the current working directory.

        if `except_blacklisted` is `True` all `directories` in
        `self.blacklisted_directories` will be removed
        """
        directories = util.to_absolute_paths(directories)
        if except_blacklisted:
            directories = self._remove_blacklisted(directories)

        self.plugin_directories.update(directories)