store platform & arch information next to installed binary

#35 made clear that, in a cross-platform-packaging project using `ffmpeg-static` as a dependency, **a second `npm install` should install an `ffmpeg` binary for a *different platform/arch than before***.

To enable this, `install.js` needs to

1. [ ] write the platform & arch next to the binary (e.g. to `ffmpeg.json`).
1. [ ] check if there's a `ffmpeg.json` in place, with a different platform/arch than desired. If platform & arch are equal, it can exit successfully.