Add a file-path function for agnostic image urls

The `file-path` function will return either a url prefixed with the
contents of the `path` variable if it is set, or a `image-url` if it is
not set (the default). This lets you use the external link icons on
projects that only use vanilla Sass.

This fixes https://github.com/alphagov/govuk_frontend_toolkit/issues/95