`build --target dir` doesn't work

[dfabulich]

https://github.com/electron-userland/electron-builder/blob/master/README.md

The README says to install a pack script like this:

"scripts": {
  "postinstall": "install-app-deps",
  "pack": "build --target dir",
  "dist": "build"
}

And then you can run npm run dist (to package in a distributable format (e.g. dmg, windows installer, deb package)) or npm run pack.

But the build command doesn't accept a target parameter, so if you actually try to npm run pack, you just get an error message, Unknown argument: target

[develar]

I think, we should add --dir to build dir target for current platform.