Yarn: feature: Implement `yarn global config set|get`

[FLGMwt]

Do you want to request a _feature_ or report a _bug_?
bug
What is the current behavior?
Currently, all yarn config set and yarn config remove commands modify home config, whether -g is set or not.
If the current behavior is a bug, please provide the steps to reproduce.
yarn config set foo bar
notice user level .yarnrc is modified
👍

yarn config set foo bar --global
notice user level .yarnrc is modified and global .yarnrc is _unmodified_
👎

What is the expected behavior?
--global should only modify global .yarnrc
Please mention your node.js, yarn and operating system version.

> node -v
v6.7.0
> npm -v
3.10.3
> yarn --version
0.15.1

[ahmedelgabri]

actually I have this the other way around.

$ yarn config set exact true

# and

$ yarn config set exact true -g

Both create and manipulate the global .yarnrc file.

I'm using 0.17.9

[BYK]

I've changed the issue since I don't think we are going to implement -g or --global flags. That would be inconsistent with how we do things. I think having a yarn global config command would be nice tho.

[sla89]

The yarn config documentation mentions the -gand --global flag for setting configuration values globally but as mentioned here this feature does not work.