zsh: show an asterisk in the prompt when there are background jobs
zsh: show an asterisk in the prompt when there are background jobs

Could also have shown the job count here (ie. `%(1j.%j.)`), but I am
generally more interested in the binary question of whether or not I
have anything at all in the background (ie. will I get a "zsh: you have
suspended jobs." error if I try to exit right now?).

Looks like this:

    wincent*$

when there are background jobs (star is bold/yellow) and:

    wincent$

when there are not.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent