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.

docs/gallery-dl.conf

@@ -3,6 +3,7 @@
     "netrc": false,
     "downloader":
     {
+        "part": true,
         "http":
         {
             "retries": 5,