store platform & arch information next to installed binary

[derhuerst]

#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).
2. 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.

[snobu]

I would suggest a naming convention for the ffmpeg binary rather than additional metadata file.

[derhuerst]

True, that would work as well. We could follow the Go or Rust binary naming scheme here.

[terribleplan]

The benefits I see with something like an ffmpeg.json or similar is that it:

1. Allows for for never having to list the directory and have conditional code for the platform as long is it is not changed between build/install time and runtime. (Store "filename": "ffmpeg.exe" to make windows just work). Else you would have to list the dir, see what is in it, find something that starts with "ffmpeg-", detect platform and arch by splitting on "-", figure out what to do if there are multiple, etc...
2. Allows for drift detection. (Store "hash": "sha256 of the file", allows for detecting if something has gotten weird/been modified/etc) Could be an interesting future feature, at least debugging tool somewhat.