Zsh: add color - to switch back to previous color

[lencioni]

If I am checking out a bunch of color schemes, I often find myself
switching to a theme for a bit and then wanting to switch back to the
one I was previously using. This commit adds this feature to the color
script by teaching it to respond to the special "-" argument. This works
similarly to how other commands like cd - and git checkout - work,
so it feels pretty natural.

I considered writing the previous color as metadata to my existing
base16 config file to avoid adding another file, but I decided that this
approach would be a little easier to implement so I am rolling with it
for now. But in the future, that might be a possible avenue for
improvement.

I originally did this in
lencioni/dotfiles@249de701 and thought you
might be interested.

[wincent]

Neat idea!

(Over-engineering instincts kicking in here make me want to implement this as a full "reflog" style thing with the ability to switch back to @{1}, @{2} etc as well. I would never use this, but keeping one previous entry seems a bit arbitrary when you could just keep a log of them, and maybe you could justify it in terms of being able to generate interesting stats on color scheme usage.)

[lencioni]

If you bring this in, you'll want this fix: lencioni/dotfiles@81f2ff6