Update mediaplayer.py with player actions

[deigil]

• To play the specified player:

  python3 waybar/scripts/mediaplayer.py --player "spotify" --action play

• To pause the specified player:

  python3 waybar/scripts/mediaplayer.py --player "spotify" --action pause

• To toggle play/pause for the specified player:

  python3 waybar/scripts/mediaplayer.py --player "spotify" --action play-pause

• To skip to the next track:

  python3 waybar/scripts/mediaplayer.py --player "spotify" --action next

• To go back to the previous track:

  python3 waybar/scripts/mediaplayer.py --player "spotify" --action previous