Further prompt/terminal configuration.

Enable transparent urxvt background. This works differently than prior
attempts, as it will do alpha-channel blending in real time for the
background colors, rather than just the root window.

Remove powerline symbols again. Like previous attempts, it's really
pretty when it works, but occasional misaligned pixels at certain font
sizes ruins the effect. After a simple background update and reboot, the
symbols misaligned again.

Change the prompt to look better without funky symbols, but retain the
striking visual effect of different background colors.