Create a command to output debug info

[tony]

Create a command to output system information to be used. This should improve quality of issues/bug reports.

Should output:

• Python version
• system PATH
• tmux version
• tmuxp version
• tmux path
• tmuxp path,
• libtmux version
• shell
• output of tmux list-sessions, tmux list-windows, tmux list-panes
• output of tmux show-options -g, tmux show-window-options -g

For use with #351

[joseph-flinn]

released in v1.6.2