Starting in detached state should also be a config-level option

[tklausmann]

As it says in the title. I have some tmuxp configs that I rarely-if-ever need to attach to (but when I need to, it's critical). It would be nice to have those default to detached startup, without having to remember to add the -d switch on the command line.

Naturally this means tmuxp be should have a switch to start attached even if the config says otherwise, but that is sorta optional.