implement cache/clean api

src/list.rs

@@ -90,4 +90,9 @@ impl List {
         self.cache.block(rule).await?;
         Ok(self.index.write().await.remove(&Item::from_line(rule)))
     }
+    /// Privately clean `--cache` file collected, return deleted rules
+    /// * we can implement `self.index` update at this step @TODO
+    pub async fn cache_clean(&self) -> Result<Option<Vec<String>>> {
+        self.cache.clean().await
+    }
 }