encode.py: Force libfdk_aac

For some reason, it is not the default.

encode.py

@@ -45,6 +45,9 @@ def encode_episode(podcast, episode, format):
         command.extend(["-i", infile])
         if format not in ["oga", "opus"]:
             command.extend(["-i", episode["poster"]])
+        # For AAC, the default codec choice (ffmpeg native) is not the best choice
+        if format == "m4a":
+            command.extend(["-c:a", "libfdk_aac"])
         command.extend(["-c:v", "copy"])
         command.extend(options)
         for k, v in tags.items():