Parse and prepare the plugin's configuration.

def setup(self, app):
        """Parse and prepare the plugin's configuration."""
        super().setup(app)

        self.enabled = len(self.cfg.backends)

        self.default = self.cfg.default
        if not self.default and self.enabled:
            self.default = self.cfg.backends[0][0]

        self.backends_hash = {name: parse.urlparse(loc) for (name, loc) in self.cfg.backends}
        if self.default and self.default not in self.backends_hash:
            raise PluginException('Backend not found: %s' % self.default)