Environment variables in tmux options not honoured

[eulersson]

See the issue https://github.com/erikw/tmux-dark-notify/issues/1 for full description of what this PR is trying to address.

[eulersson]

@erikw Feel free to tackle this in any way you want... I you want to discard the PR it's OK but some users, specially on macOS might face the issue of not seeing the change reflected because of the wrong variable expansion.

[eulersson]

Thanks for your plugin by the way! It's very nice :)