Add exit code display to the custom prompt

When experimenting with commands for shell scripting, it is often very
useful to have the exit code of commands that fail displayed in the
prompt. This allows one to see what exit code different commands return
for different errors. Since the exit code is just noise most of the
time, it is set to only display when a non-zero exit code is
returned. It also displays in red so as to make it visually separate
from the other elements of the prompt.
Add a guard clause to ensure that rbenv gets loaded

On my OS X machines, rbenv is installed by Homebrew which adds rbenv to
/usr/local/bin on the PATH. On my Ubuntu machines (specifically the
Vagrant image for my development projects), rbenv is just installed by
git cloning the project into the home directory ... which doesn't place
the rbenv binary onto the PATH. This guard clause checks to see if rbenv
is available and not yet loaded and loads it.