[YoutubeDL] Simplify API of YoutubeDL

Calling add_default_extractors twice should be harmless since the first set of extractors will match.

youtube_dl/YoutubeDL.py

@@ -189,7 +189,7 @@ class YoutubeDL(object):
     _num_downloads = None
     _screen_file = None
 
-    def __init__(self, params=None):
+    def __init__(self, params=None, auto_init=True):
         """Create a FileDownloader object with the given options."""
         if params is None:
             params = {}
@@ -246,6 +246,10 @@ class YoutubeDL(object):
 
         self._setup_opener()
 
+        if auto_init:
+            self.print_debug_header()
+            self.add_default_info_extractors()
+
     def add_info_extractor(self, ie):
         """Add an InfoExtractor object to the end of the list."""
         self._ies.append(ie)