Generate storyshots in file-server mode

Part of #98 (closed)

Due to "cross domain" security precautions, browsers will refuse to load and render external SVGs referenced from a file:// url. This means that SVG sprites, like those employed by the forthcoming <icon> component will not show up when generating visual regression tests through storyshots, because this loads filesystem URLs into Chrome via puppeteer rather than serving storybook over a fileserver.

We need to update our storyshots config to utilize a local fileserver rather than file:// based URLs.