feed: always downloads with resume

what if the server doesn't support range? we don't support it!

feed

@@ -396,6 +396,18 @@ def get_parser():
     return p
 
 
+def downloader(url, dest):
+    headers = {}
+    if os.path.exists(dest):
+        headers["Range"] = "bytes=%d-" % os.stat(dest).st_size
+    r = requests.get(url, stream=True, headers=headers)
+    if r.status_code == 416:  # range not satisfiable
+        return
+    with open(dest, "ab") as f:
+        for chunk in r.iter_content(chunk_size=2 << 15):
+            f.write(chunk)
+
+
 def put(audio, copy=False):
     if not copy:
         for url in audio.urls:
@@ -411,7 +423,7 @@ def put(audio, copy=False):
                 ).rstrip()
                 dest = os.path.join(destdir, fname)
                 os.makedirs(destdir, exist_ok=True)
-                fname, headers = urllib.request.urlretrieve(url, dest)
+                downloader(url, dest)
                 print("file://%s" % os.path.realpath(fname))
             else:
                 # FIXME: file:// urls are just copied