Config file, player definition with environment variable/value stored in the external file.#383

Hi, Sometime ago I wanted to have [gapless playback](https://github.com/ZaxonXP/youtube-viewer-scripts), so I found a way that I can play one audio in one instance of mpv, then run in pause mode the other mpv with the next audio. When the first one ended, the next one would play immediately. For that I had to use two separated sockets (`/tmp/soc1`, `/tmp/soc2`) to be able to send pause command. The info about currently used socket I stored in a third file (`/tmp/soc_used`). This also allows me to globally control the playback (play/pause, next, etc.). I know, nothing to do with the `youtube-viewer` so far, but it should allow you to understand the rest. :) So now, I would like to use this idea with `youtube-viewer`, but I do not know how to get the content of the `/tmp/soc_used` and place it in the `youtube-viewer.config` file? The content of the `/tmp/soc_used` would be either `/tmp/soc1` or `/tmp/soc2`. Alternatively I could read the content of `/tmp/soc_used` and store it in the shell variable. But then, how can I define it in the `youtube-viewer.config` file? Can I use the `$ENV{VAR_NAME}` in the config? Will it be expanded to the proper value? Can the shell exported variables be used inside the config file? Kind regards, Piotr