Disable auto-loading PostCSS config file

[egoist]

Since postcss-loader does not load it when option is set. https://github.com/postcss/postcss-loader/blob/master/index.js#L67

[michael-ciniawsky]

@egoist I work on a PR already including a few other smaller improvements and yes I would also change it to that logic. inline ? use(inline) : use('postcss.config.js') // inline = webpack.config.js. So it's either explicit via webpack.config.js or implicit via postcss.config.js, but clearly separated.