Issue title: how do I turn off autoprefixing "grid" rule?
 Issue body: `  postcss: function (webpack) {
    return [
      require("postcss-import")({ addDependencyTo: webpack }),
      require("postcss-cssnext")({
        browsers: ['last 2 versions'],
        feature: {
          grid: false
        }
      }),
      require("postcss-reporter")()
    ]
  },`

it looks like `grid: false` being ignored
 Comments: 
 Comment 0: ```js
postcss: function (webpack) {
    return [
      require("postcss-import")({ addDependencyTo: webpack }),
      require("postcss-cssnext")({
        browsers: ['last 2 versions'],
        feature: {
          autoprefixer: { grid: false }
        }
      }),
      require("postcss-reporter")()
    ]
  }
```
 Comment 1: If anyone else comes across this, I believe the correct config should be as follows:

```js
features: {
  autoprefixer: { grid: false }
}
```

note `features` instead of `feature`. Also, I believe grid is disabled by default, so you really only need this config if you want to enable grid prefixing (like I do), which would be `grid: true` instead.
{'source': 'GithubIssues', 'repo_name_with_owner': 'MoOx/postcss-cssnext', 'issue_id': 140806769, 'repo_stars': 3760.0}