Remove static/ example:

This was removed because it requires you to mutate the Python package
to do it. There's currently no way to do the same thing using ~/.grip
so removing this until including local styles becomes a need.
Rename CSS style.
Remove leftover STYLE_URLS note.
Clarify Configuration section and add an example.
Add "GitHub rate limit reached" page & preview.
Bypass GitHub's image cache:

The problem appears to be that GitHub replaces all external images with a cache
stored on their servers. When using the Markdown API, these image URLs are still
replaced, but image doesn't always resolve. The effect is that sometimes images
are shown, and sometimes not. This is probably why #50 appeared to be resolved.

This fix makes sure the image is shown from the source, so it'll always
resolve (as long as it still exists on the original servers). A slightly better
fix would be to only handle images that error out, but that optimization will
be in a future fix.