[postprocessor] Add plugin support

Adds option `--use-postprocessor` to enable them

ytdlp_plugins/extractor/__init__.py

@@ -1,3 +1,4 @@
-# flake8: noqa
+# flake8: noqa: F401
 
+# ℹ️ The imported name must end in "IE"
 from .sample import SamplePluginIE

ytdlp_plugins/extractor/sample.py

@@ -1,7 +1,5 @@
 # coding: utf-8
 
-from __future__ import unicode_literals
-
 # ⚠ Don't use relative imports
 from yt_dlp.extractor.common import InfoExtractor
 

ytdlp_plugins/postprocessor/__init__.py

@@ -0,0 +1,4 @@
+# flake8: noqa: F401
+
+# ℹ️ The imported name must end in "PP" and is the name to be used in --use-postprocessor
+from .sample import SamplePluginPP

ytdlp_plugins/postprocessor/sample.py

@@ -0,0 +1,23 @@
+# coding: utf-8
+
+# ⚠ Don't use relative imports
+from yt_dlp.postprocessor.common import PostProcessor
+
+
+# ℹ️ See the docstring of yt_dlp.postprocessor.common.PostProcessor
+class SamplePluginPP(PostProcessor):
+    def __init__(self, downloader=None, **kwargs):
+        # ⚠ Only kwargs can be passed from the CLI, and all argument values will be string
+        # Also, "downloader", "when" and "key" are reserved names
+        super().__init__(downloader)
+        self._kwargs = kwargs
+
+    # ℹ️ See docstring of yt_dlp.postprocessor.common.PostProcessor.run
+    def run(self, info):
+        filepath = info.get('filepath')
+        if filepath:  # PP was called after download (default)
+            self.to_screen(f'Post-processed {filepath!r} with {self._kwargs}')
+        else:  # PP was called before actual download
+            filepath = info.get('_filename')
+            self.to_screen(f'Pre-processed {filepath!r} with {self._kwargs}')
+        return [], info  # return list_of_files_to_delete, info_dict