implement cache/clean api

src/list/cache.rs

@@ -49,6 +49,22 @@ impl Cache {
         }
         Ok(())
     }
+    /// Clean `--cache` file collected, return deleted rules
+    pub async fn clean(&self) -> Result<Option<Vec<String>>> {
+        match self.0 {
+            Some(ref p) => {
+                init_file(p).await?;
+                let mut rules = Vec::new();
+                let lines = fs::read_to_string(p).await?;
+                for line in lines.lines() {
+                    rules.push(line.into());
+                }
+                fs::write(p, "").await?;
+                Ok(Some(rules))
+            }
+            None => Ok(None),
+        }
+    }
 }
 
 /// Make sure that cache file is always exist (e.g. user may remove it when the daemon is running)