Fix bug where the shell environment was not initialised when
doing a horizontal or vertical windows split.
Used more sensible way to set the default shell command when creating
windows. Now it doesn't matter if the user uses non-default
key-bindings.
Add attach command to init-tmux that prompts the user to attach to
a tmux session that we manage.
When setting default-command option do so quietly so ``init-tmux start``
command is cleaner.
Improve error message when an incorrect command is chosen in init-tmux