Add fallback downloader

bdfr/site_downloaders/fallback_downloaders/fallback_downloader.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+# coding=utf-8
+
+from abc import ABC, abstractmethod
+
+from bdfr.site_downloaders.base_downloader import BaseDownloader
+
+
+class BaseFallbackDownloader(BaseDownloader, ABC):
+
+    @staticmethod
+    @abstractmethod
+    def can_handle_link(url: str) -> bool:
+        """Returns whether the fallback downloader can download this link"""
+        raise NotImplementedError