Adds a strip index file

This should reduce the amount of time we spend on querying for comics that are already present

__main__.py

@@ -71,11 +71,14 @@ if __name__ == "__main__":
     logging.info("Beginning parsing")
         
     threads = []
+    comics = []
     thread_limit = threading.BoundedSemaphore(value=args.threads)
     
     for url in args.url:
         comic_type = find_comic_class(url.netloc + url.path)
         comic = comic_type.create_from_url(url)
+        comics.append(comic)
+        comic.read_index(base_path)
         comic.await_load()
         logging.info("There are %d comics", comic.latest_identifier)
         if args.latest:
@@ -85,11 +88,19 @@ if __name__ == "__main__":
             start = args.start or comic.get_first_strip()
             end = args.end or comic.get_latest_strip()
             r = r[start - 1:end]
-    
-        for index in reversed(r):
-            t = threading.Thread(name=str(comic.get_identifier_string()) + "-" + str(index), target=load_strip, args=(base_path, comic, index, args.plain, thread_limit))
-            threads.append(t)
-            t.start()
 
-    for t in threads:
-        t.join()
+        try:
+            for index in reversed(r):
+                if not comic.is_strip_present(base_path, ".cbz", str(index)):
+                    t = threading.Thread(name=str(comic.get_identifier_string()) + "-" + str(index), target=load_strip, args=(base_path, comic, index, args.plain, thread_limit))
+                    threads.append(t)
+                    t.start()
+        finally:
+            comic.save_index(base_path)
+
+    try:
+        for t in threads:
+            t.join()
+    finally:
+        for comic in comics:
+            comic.save_index(base_path)