[YoutubeDL] Make postprocessors declarative

Instead of having to configure PPs in code, this allows us and embedding programs not to worry about imports or finer details, similarly to how we handle IEs.

youtube_dl/postprocessor/__init__.py

@@ -8,11 +8,16 @@ from .ffmpeg import (
     FFmpegExtractAudioPP,
     FFmpegMergerPP,
     FFmpegMetadataPP,
-    FFmpegVideoConvertor,
+    FFmpegVideoConvertorPP,
 )
 from .xattrpp import XAttrMetadataPP
 from .execafterdownload import ExecAfterDownloadPP
 
+
+def get_postprocessor(key):
+    return globals()[key + 'PP']
+
+
 __all__ = [
     'AtomicParsleyPP',
     'ExecAfterDownloadPP',
@@ -22,6 +27,6 @@ __all__ = [
     'FFmpegMergerPP',
     'FFmpegMetadataPP',
     'FFmpegPostProcessor',
-    'FFmpegVideoConvertor',
+    'FFmpegVideoConvertorPP',
     'XAttrMetadataPP',
 ]