Apply remain-on-exit tmux option only to windows created by overmind

[jerryskye]

Hello 👋 thank you for creating overmind, I really appreciate this tool 🥇
I'm an avid tmux user and I usually run overmind as a daemon. I start my web server and stuff, run overmind connect and do all my work right there in that tmux session where I can have everything in one place thanks to overmind 🙂

However, I noticed the remain-on-exit tmux option being set globally using setw -g and it's making all the windows I open inherit this setting. Looks like it's possible to make remain-on-exit only apply to windows that overmind created with this simple change I made. I imagine this would save the trouble of dealing with Pane is dead messages for someone looking at some logs with overmind connect and quickly wanting to do something in the same tmux session 😁
The -p flag restricts this setting to the pane, so it'll make it possible to even do some work in a split pane in the same windows as process run by overmind.

Thanks again and let me know if you'd be open to this change 😊

[dlight]

Hello, is there anything blocking merging this PR?