Add startup_pane option to sample configuration.
Add startup_pane option to sample configuration.

- Added a startup pane option to the sample yml file.
- Modified the startup_window to the editor window, so the startup pane
  option can actually select one out of two panes in the selected
window.

pczarn/tmuxinator,ethagnawl/tmuxinator,tmuxinator/tmuxinator,ethagnawl/tmuxinator,podung/tmuxinator,adamstrickland/tmuxinator,tmuxinator/tmuxinator,klaxalk/tmux,klaxalk/tmux,adamstrickland/tmuxinator,klaxalk/tmux,podung/tmuxinator,ethagnawl/tmuxinator,klaxalk/tmux,podung/tmuxinator,klaxalk/tmux,adamstrickland/tmuxinator,tmuxinator/tmuxinator,J3RN/tmuxinator,pczarn/tmuxinator,J3RN/tmuxinator,pczarn/tmuxinator,J3RN/tmuxinator