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
Remove trailing newline escapes.

The previously-last line was removed in 0f091fe, but we forgot to remove the
escape that says we're done - oops!
Merge branch 'misc-cleanups'

Some minor cleanups. The more important changes are the $TMUX check and more
accurately keeping focus on the correct pane.

Closes #1.