New behavior of `new_window` helper:
- When no argument is specified no specific window name is set.
- Second argument added to pass a shell command to be executed.
If a shell command is needed, but no window name is desired, pass an
empty string `""` as the first argument.
Additionally, also fix a bug when creating a window, where as if the
session name was the same as a window that already existed, it would
fail, accidentally using the window rather than the session at a target.