Eliminate space counted as message length (#126)

Co-authored-by: William Woodruff <william@yossarian.net>

ff2mpv.py

@@ -49,7 +49,11 @@ def get_message():
 
 
 def send_message(message):
-    content = json.dumps(message).encode("utf-8")
+    # https://stackoverflow.com/a/56563264
+    # https://docs.python.org/3/library/json.html#basic-usage
+    # To get the most compact JSON representation, you should specify
+    # (',', ':') to eliminate whitespace.
+    content = json.dumps(message, separators=(",", ":")).encode("utf-8")
     length = struct.pack("@I", len(content))
     sys.stdout.buffer.write(length)
     sys.stdout.buffer.write(content)