Add extra option for base pane index for tmuxinator

I had issues with tmuxinator not running commands inside of a multi-paned window when I started a new tmux session. Here's a blip of one of my tmuxinator session setup files:

```yml
windows:
  - editor:
      layout: 8087,238x60,0,0[238x44,0,0,0,238x15,0,45,5
      panes:
        - vim
        -
```

I could never get the `vim` command (as well as others not shown here) to auto-run in the pane, until I made this change to the dotfiles. I got it off of the [tmuxinator readme](https://github.com/tmuxinator/tmuxinator#base-index), and figured it might be helpful to include it here for more out-of-the-box support