Unset TMUX before splitting window
Unset TMUX before splitting window

Avoids confusing grouped sessions.
Fixes #643

janlazo/fzf,tomfitzhenry/pkg-fzf,lululau/fzf,lululau/fzf,wilywampa/fzf,junegunn/fzf,infokiller/fzf,lululau/fzf,junegunn/fzf,tomfitzhenry/pkg-fzf,wilywampa/fzf,tomfitzhenry/pkg-fzf,janlazo/fzf,junegunn/fzf,wilywampa/fzf,stuha/wbr-zsh,janlazo/fzf,infokiller/fzf,infokiller/fzf