[postprocessor/ffmpeg] Allow options for mp3

Why?
Options like `preferredquality` are ignored when postprocessing mp3s.

What?
Remove `mp3` from the list of `filecodec` values to check for in the
"lossless" condition unless `preferredquality` is unspecified.

youtube_dl/postprocessor/ffmpeg.py

@@ -284,7 +284,10 @@ def run(self, information):
                 acodec = 'copy'
                 extension = 'm4a'
                 more_opts = ['-bsf:a', 'aac_adtstoasc']
-            elif filecodec in ['aac', 'flac', 'mp3', 'vorbis', 'opus']:
+            elif (
+                filecodec in ['aac', 'flac', 'vorbis', 'opus'] or
+                (filecodec == 'mp3' and self._preferredquality is None)
+            ):
                 # Lossless if possible
                 acodec = 'copy'
                 extension = filecodec