Add new-window command functionality #142
Description
Firstly, this project is pretty nice.
One feature of tmux I use frequently is the ability to start a new window with a command. For instance, this is helpful with working in virtual environments in python. I typically run new-window -n name 'pew workon venv_name' to launch a new window with the virtual environment activated. This way, I don't have to exit twice to exit the window. When I exit the venv, the command quits, and so does the window with it.
Other than that, you seem to have quite a lot of tmux's functionality covered. Good job.