Allow passing postprocessor class directly

This makes it possible to create custom postprocessors easily

youtube_dl/postprocessor/__init__.py

@@ -16,12 +16,13 @@ from .ffmpeg import (
 from .xattrpp import XAttrMetadataPP
 from .execafterdownload import ExecAfterDownloadPP
 from .metadatafromtitle import MetadataFromTitlePP
-
+from .common import PostProcessor
 
 def get_postprocessor(key):
+    if isinstance(key, PostProcessor):
+        return key
     return globals()[key + 'PP']
 
-
 __all__ = [
     'EmbedThumbnailPP',
     'ExecAfterDownloadPP',