Add danger zone tmux behavior

[alanchrt]

I think it would be nice to look for a local file ~/.danger that could be executed while tmux runs. If it returned a non-zero exit status, the tmux status bar would flash on and off to red.

The use case is this: if I'm in production kubectl context or I'm in a production terraform workspace, I want to know, always. Warnings in the prompt are nice, but it would be hard to miss a flashing status line until I'd left the prod context.