Add `.file()` method

[Tobias Bieniek]

I think it would be useful to add a path() method that did something like:

function path(path, options) {
  return new Promise((resolve, reject) => {
    let stream = fs.createReadStream(path);
    stream.on('error', reject);
    stream.pipe(gzipSize.stream(options)).on('gzip-size', resolve);
  });
}

Would that be a welcome PR?

[Sindre Sorhus]

Yes, that would be welcomed. Should be named .file() though, and remember to also add an error handler on the gzipSize.stream (Errors doesn't propagate in Node.js streams...).