respect global option for config add and config delete

[FLGMwt]

Summary
Global option for yarn config is documented, but unimplemented.

Test plan

> yarn config list
yarn config v0.15.1
info yarn config
{ 'version-tag-prefix': 'v',
  'version-git-tag': true,
...
PS C:\Program Files\nodejs> yarn config set ".some-key" "user value"
yarn config v0.15.1
success Set ".some-key" to "user value".
Done in 0.03s.
PS C:\Program Files\nodejs> yarn config set ".some-key" "global value" -g
yarn config v0.15.1
success Set ".some-key" to "global value".
Done in 0.04s.
PS C:\Program Files\nodejs> cat .\.yarnrc
# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
# yarn lockfile v1
".some-key" "global value"
PS C:\Program Files\nodejs> cat ~/.yarnrc
# THIS IS AN AUTOGENERATED FILE. DO NOT EDIT THIS FILE DIRECTLY.
# yarn lockfile v1
".some-key" "user value"
> yarn config list
yarn config v0.15.1
info yarn config
{ '.some-key': 'global value',
...

[FLGMwt]

FYI example output shows global value favored over user value. I address this in #951

[FLGMwt]

Fixes #1037

[bestander]

I think we have yarn global add/remove since then.
Sorry for loosing this