Add a VimuxPaneSetup configuration setting

[Julian]

Hey, thanks for a great plugin.

I think it'd be nice to have a way to specify a bunch of stuff to be run when a pane is first created by Vimux. For me the use case here is that I have a large prompt (3 lines), and I use an envvar to designate to my shell that I want to instead use a more micro prompt. I want that prompt to be used by Vimux (and currently activate that within my Vimux commands), but it'd be nicer I think to be able to just specify stuff once globally rather than needing to put that in each of my commands.

(I know VimuxResetSequence exists, but that doesn't really seem appropriate to do this kind of thing).

[alerque]

If I understand your use case correctly, I think what you should do is set the env var in the tmux client invocation, something like this:

let g:VimuxTmuxCommand = "MYENVVAR=value tmux"

This should get the variable set in the environment before the shell command even runs.

If that doesn't do the trick or cover some use case I'd be happy to accept a contribution with a setup function like you mentioned as a PR. Feel free to comment if this doesn't work and we can re-open to track the need.