Simplify Airline config for Powerline symbols.

The private use space symbols Powerline uses don't have to be listed
manually. Instead, use the boolean option to automatically add them.
Simplify ZSH prompt coloring.

Use true black for the background text in a few cases. Using a different
terminal color scheme would cause problems with 16 selected (part of the
256 color space). Use a 16-color mode red for the background of error
result prompts.