Support -S/--save flags for yarn add

[avaly]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
Running yarn add -S throws an error and does not install a package. This should be supported to ease the transition for npm users.

If the current behavior is a bug, please provide the steps to reproduce.
Run yarn add -S babel or yarn add --save babel

Actual output

yarn add v0.19.1
error Missing list of packages to add to your project.
info Visit https://yarnpkg.com/en/docs/cli/add for documentation about this command.

What is the expected behavior?
It should install the package and add it to dependencies in package.json.

Please mention your node.js, yarn and operating system version.
node v6.9.4
yarn v0.19.1
Ubuntu 16.04

[torifat]

https://yarnpkg.com/en/docs/migrating-from-npm#toc-cli-commands-comparison

[avaly]

I still think this flag could be useful to support without such a hard fail.