add options to control usage of .part files (#29)

- '--no-part' command line option to disable them
- 'downloader.http.part' and 'downloader.text.part' config options

Disabling .part files restores the behaviour of the old downloader
implementation.

gallery_dl/option.py

@@ -145,6 +145,11 @@ def build_parser():
         metavar="SECONDS", action=ConfigAction, dest="timeout", type=float,
         help="Timeout for HTTP connections (defaut: 30s)",
     )
+    downloader.add_argument(
+        "--no-part",
+        action=ConfigConstAction, nargs=0, dest="part", const=False,
+        help="Do not use .part files",
+    )
     downloader.add_argument(
         "--abort-on-skip",
         action=ConfigConstAction, nargs=0, dest="skip", const="abort",