Library for loading files

[curiousdannii]

If media is ever to be supported in Hugo or TADS, we'll need a way to load additional resource files that sit alongside the main storyfile. This is a little bit complicated as it could be from the file system or across the internet (using the proxy if necessary). It should be async from the start (so probably depends on erkyrath/glkote#40).

I was originally thinking this should be part of the Dialog library, but I just had the thought that maybe there should be a mini library to handle loading and possible caching files (which I wanted as part of Parchment: curiousdannii/parchment#99).