Allow appearance to be customizable per-machine

To do this, I moved most of init-appearance into site-init and made an
example file so that I have a place to start from.

It is worth noting that I treat all themes as safe.  This .emacs comes
with a deftheme clone of jellybeans.vim that I made that I quite like;
if you want to audit it you can look at colors/jellybeans-theme.el.  It
does not come with any other themes (though you could install them
using (use-package [theme] :ensure t) in your site-init).

At some future point I may move the 'treat themes as safe' into
site-init.
Remove Quicklisp stuff from example site-init