title: Running a command for every new window in tmux
body: <p>I wrote <a href="https://github.com/mqsoh/tenper" rel="nofollow">this thing</a> and I'd like to source a Python virtualenv activate script whenever a new window is created in a running session.</p>
  
  <p>Is it possible? The default-command option is related to the shell, not an additional command.</p>