Instead of weirdly smearing the color, simply increase the values
until they are bright enough.
This prevents /tmp from being white, and guarantees visible colors for
all directories.
Some features:
- A nice `►` prompt char with a fallback for non-utf8 systems
- The $PWD is colored depending on its sha, so different directories
are colored differently, but each directory stays the same
- User@Host is only shown if not on the local machine (ssh or
virtualization)
- A right prompt with a nice git display, date, duration of the last
command (if it took over 100ms), and virtualenv
