Add support for custom player from env var

[eSoares]

This adds support to easily define to the user a custom player without need to edit the scripts.
Not changing the scripts makes it easy to simple git pull to keep them updated.

[marmelo]

Hi @eSoares, sorry for the late response.

I understand the need to change the player.
But there are some arguments specific to mpv that may not work on other players.

That said, I suggest a simpler approach using Bash parameter expansion to set a player if an environment variable is not found. That requires the following simple change to the script:

PLAYER=${PLAYER:-mpv}

This way you can do:

PLAYER=mplayer ./radio.sh

What do you think?