FR: Add Xephyr custom arguments

I want to add a few arguments to Xephyr, for example -dpi, -resizable or -scale 2x2, but it's not possible as it is. So what if we use environment variables, for example `XEPHYR_ARGS="-blah -blah2"` and later ``` Xephyr -noreset -ac -br $XEPHYR_ARGS -screen "${XEPHYR_SIZE}" :"${xephyr_display}" &>/dev/null & sleep 1 ```