Not loading config.path or default postcss.config.js

When I run `postcss -c postcss.config.js`, my css is processed.

When I run Rollup with this plugin, it is not:

```
postcss({
config: {
path: __dirname + 'postcss.config.js'
}
})

```

node -v = v12.4.0
npm -v 6.9.0
OSX 10.14.5 (18F132)

```
"postcss-cli": "2.6.0",
"postcss-cssnext": "2.10.0",
"postcss-discard-comments": "2.0.4",
"postcss-discard-empty": "2.1.0",
"postcss-import": "8.1.2",
"postcss-reporter": "1.4.1",
"rollup-plugin-postcss": "^2.0.3",