BUG: MPV playlist arguments are out-of-date

[acdw]

Hello devs and users!

I installed EMMS the other day and was trying to play and set up streaming 
playlists.  However, they wouldn't play at first.  I traced the issue to 
`emms-player-mpv-start' in emms-player-mpv.el:

;;...
    (if (emms-player-mpv-ipc-fifo-p)
        (progn
          ;; ipc-stop is to clear any buffered commands
          (emms-player-mpv-ipc-stop)
          (emms-player-mpv-proc-init (if track-is-playlist "--playlist" "--")
;;                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 
here
                                     track-name)
          (emms-player-started emms-player-mpv))
;;...

I'm not sure when, but my version of MPV (0.32.0) doesn't take a plain 
--playlist argument; rather it says this about the --playlist argument:
...

       --playlist=<filename>
              Play  files  according  to a playlist file (Supports some common
              formats. If no format is detected, it will be treated as list of
              files,  separated  by newline characters. Note that XML playlist
              formats are not supported.)

              You can play playlists directly and without  this  option,  how‐
              ever, this option disables any security mechanisms that might be
              in place. You may also need this option to load plaintext  files
              as playlist.

              WARNING:
                 The  way  mpv  uses playlist files via --playlist is not safe
                 against maliciously constructed files. Such files may trigger
                 harmful  actions.   This  has  been  the case for all mpv and
                 MPlayer versions, but unfortunately this fact  was  not  well
                 documented  earlier,  and  some  people have even misguidedly
                 recommended use of --playlist with untrusted sources. Do  NOT
                 use  --playlist  with random internet sources or files you do
                 not trust!

                 Playlist can contain entries using other protocols,  such  as
                 local files, or (most severely), special protocols like avde‐
                 vice://, which are inherently unsafe.

...
Due to the changed syntax of the --playlist argument, coupled with the warning 
in MPV's manual, I changed the above highlighted form to simply "--" in 
`emms-player-mpv-start'.  With my minimal testing it's working fine, though to 
change the code in the package it probably needs more testing.

Let me know what I can do to help!

-- 
~ acdw
acdw.net | breadpunk.club/~breadw