added g:VimuxUseExistingPaneWithIndex configuration

I use vimux in MacVim and want to run my rspec tests always in the first pane of my current tmux session. This options enables me to do so.

To enable that setting only in MacVim and disable it for the terminal I added the following to my .vimrc.

```
if has("gui_running")
  let g:VimuxUseExistingPaneWithIndex = 1
end
```