[options] Add --mtime option, unsets default --no-mtime

* resolves #1709 (!)

youtube_dl/options.py

@@ -731,9 +731,13 @@ def parseOpts(overrideArguments=None):
         '--no-part',
         action='store_true', dest='nopart', default=False,
         help='Do not use .part files - write directly into output file')
+    filesystem.add_option(
+        '--mtime',
+        action='store_true', dest='updatetime', default=True,
+        help='Use the Last-modified header to set the file modification time (default)')
     filesystem.add_option(
         '--no-mtime',
-        action='store_false', dest='updatetime', default=True,
+        action='store_false', dest='updatetime',
         help='Do not use the Last-modified header to set the file modification time')
     filesystem.add_option(
         '--write-description',