In-memory file cache in actix-files

It would be nice if there was a feature in actix-files that allows reading static files into memory at startup and serving them from there, instead of reading them from disk. Currently, [actix-web-static-files](https://github.com/kilork/actix-web-static-files) allows to read static files from memory, but it embeds static files into the executable, instead of loading them into memory at runtime. There's probably no need to refresh the files (re-read them from disk) once they're loaded into memory, as static files are rarely updated.