Most configuration in postcss.config.js is ignored

The parser property in postcss.config is completely ignored.

Here's an example **postcss.config.js** of what I'd like to do:
```js
module.exports = {
parser: 'postcss-scss',
plugins: [
...
]
}
```

Actually looking at the code, it seems like only the plugins array is used. This is the only reference to `config` in the whole file:

https://github.com/mhanberg/jekyll-postcss/blob/90d09573b16d2ad5d6b9c3d902dadf0e14e8ac81/bin/postcss#L7-L9

For those looking for a quick-and-dirty workaround, [additional options](https://postcss.org/api/#processoptions) can be added to the [process call](https://postcss.org/api/#processor-process) here:

https://github.com/mhanberg/jekyll-postcss/blob/90d09573b16d2ad5d6b9c3d902dadf0e14e8ac81/bin/postcss#L10

Start at bin/postcss, especially the configuration handling around the referenced lines, and compare the current use of the plugins array with the PostCSS process call. Trace how postcss.config.js is loaded and determine how the parser and other process options should be honored. Done means supported configuration properties are no longer silently ignored.

Written by the indexing model from the issue text.