Version at 0.1.
Use a die function
Check for tmux properly

We don't want to try and split tmux inside of screen. It was also
missing support for screen-256color.
Don't destroy the current layout for the window
Fix pane focusing by not selecting the new pane

The wrong pane would be selected if a pane in the bottom half of a
window is the origin of the command.
Remove trailing newline
Close the pane when the log is closed

The user shouldn't have to deal with a shell in a tool-spawned pane.