Enable remote image cache for prawn-svg

[mojavelinux]

Enable the remote image cache for prawn-svg so that remote images are only downloaded once per document. Alternatively, we may want to add integration with open-uri-cached so that these images are cached between runs as well (with smart update detection). We're shooting for the most optimal behavior.

[mojavelinux]

We should also link this cache between the main and scratch document to avoid having two caches. The same is true for the raster image cache (which is already enabled by default).

[mojavelinux]

We can start by simply enabling the option if the cache-uri attribute is set. I think prawn-svg should integrate it's cache with open-uri/cached, or expose the cache methods. Otherwise, I don't see a way for a deeper integration.