[postprocessor] Add plugin support

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

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