work with base-index that isn't zero

In 0.6, you removed the explicit base-index setting, but you didn't update the ConfigWriter to work with any base-index other than zero. So right now, if you set base-index to 1, for example, tmuxinator breaks and can't open the second window that you have configured in your tmuxinator session. This fixes tmuxinator to actually use the base-index that tmux knows about.