Move to postcss org

[MoOx]

Following segmentio/myth#100
It would be nice to move this repo to https://github.com/postcss
So we can move forward with this plugin.
I want to fix opened issue, & make it a real polyfill (ref reworkcss/rework-vars#30)
And add tests, because untested code is not reliable.

[MoOx]

FYI, I've published a repo/package correctly tested https://github.com/postcss/postcss-custom-properties
But I look forward to deprecate it as soon as you will be open to move this repo so we can merge them.

[iamvdo]

Why you want to deprecate it if it's more robust than this one? I rather prefer deprecate this one instead, and keep postcss-custom-properties (the name is better IMO).

[MoOx]

To keep one name only. But works for me if you are ready to deprecated this one. Make sense.

[MoOx]

Since you are not using postcss-vars anymore, you should use npm deprecate & update README to redirect potential users to new repo.
I'm dropping reference of this repo from the custom-prop repo