[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/YoutubeDL.py

@@ -27,6 +27,7 @@ from .compat import (
     compat_cookiejar,
     compat_expanduser,
     compat_http_client,
+    compat_kwargs,
     compat_str,
     compat_urllib_error,
     compat_urllib_request,
@@ -67,7 +68,11 @@ from .cache import Cache
 from .extractor import get_info_extractor, gen_extractors
 from .downloader import get_suitable_downloader
 from .downloader.rtmp import rtmpdump_version
-from .postprocessor import FFmpegMergerPP, FFmpegPostProcessor
+from .postprocessor import (
+    FFmpegMergerPP,
+    FFmpegPostProcessor,
+    get_postprocessor,
+)
 from .version import __version__
 
 
@@ -176,6 +181,11 @@ class YoutubeDL(object):
     extract_flat:      Do not resolve URLs, return the immediate result.
                        Pass in 'in_playlist' to only show this behavior for
                        playlist items.
+    postprocessors:    A list of dictionaries, each with an entry
+                       key:  The name of the postprocessor. See
+                             youtube_dl/postprocessor/__init__.py for a list.
+                       as well as any further keyword arguments for the
+                       postprocessor.
 
     The following parameters are not used by YoutubeDL itself, they are used by
     the FileDownloader:
@@ -256,6 +266,13 @@ class YoutubeDL(object):
             self.print_debug_header()
             self.add_default_info_extractors()
 
+        for pp_def_raw in self.params.get('postprocessors', []):
+            pp_class = get_postprocessor(pp_def_raw['key'])
+            pp_def = dict(pp_def_raw)
+            del pp_def['key']
+            pp = pp_class(self, **compat_kwargs(pp_def))
+            self.add_post_processor(pp)
+
     def warn_if_short_id(self, argv):
         # short YouTube ID starting with dash?
         idxs = [