postcss-import config missing options

In the config object of `postcss-import`, there's a **path** option to define where it has to search for a file.

at the moment the only way is to create a resolve function:
```js
const path = require('path');
const atImport = require('postcss-import');
const postcssNormalize = require('postcss-normalize');

module.exports = {
// [...]
plugins: [
atImport(postcssNormalize(/* opts */).postcssImport({
resolve: (id) => path.resolve(__dirname, 'path/to/css', `${id}.css`),
})),
],
};
```

but we could be allowed to use **options.path**

```js
const atImport = require('postcss-import');
const postcssNormalize = require('postcss-normalize');

module.exports = {
// [...]
plugins: [
atImport(postcssNormalize(/* opts */).postcssImport({
path: 'path/to/css',
})),
],
};
```

Search the repository for postcss-import configuration handling and related tests, then start at the entry point that passes options to the plugin. Use the issue examples to determine the expected path behavior. Done means options.path works without a custom resolve function and the relevant tests pass.

索引モデルが issue の本文から書いたものです。