Minor tweaks (#154)

* Minor tweaks

Use at your own risk. Just thought I’d contribute at least a little bit

* Exit code 0

Exit code 0 is standard for notifying the system of a regular, non-error exit.

* No longer need unnecessary flag

* Fix `already_tagged` not defined

* <3 Zen of Python

* Silly me

core/misc.py

@@ -127,7 +127,7 @@ def filter_path(path):
 
 def grace_quit():
     print('\n\nExiting.')
-    sys.exit()
+    sys.exit(0)
 
 def get_sec(time_str):
    v = time_str.split(':', 3)

spotdl.py

@@ -236,10 +236,8 @@ def write_album(album):
 
 def download_song(file_name, content):
     """Download the audio file from YouTube."""
-    if args.input_ext == '.webm':
-        link = content.getbestaudio(preftype='webm')
-    elif args.input_ext == '.m4a':
-        link = content.getbestaudio(preftype='m4a')
+    if args.input_ext in (".webm", ".m4a"):
+        link = content.getbestaudio(preftype=args.input_ext[1:])
     else:
         return False