Ability to pass custom headers with page remote CSS requests

My use case involves requesting remote CSS assets from servers that require specific HTTP headers in place to allow requests to proceed.

I'd like to extend the [`requestAsync()`](https://github.com/addyosmani/critical/blob/master/lib/file-helper.js#L131) method to respect a configuration option (e.g. `assetHeaders`) to provide a map of custom HTTP headers when requesting assets.

e.g. usage
```
critical.generate({
inline: false,
base: '.',
folder: '.',
html: '...',
minify: true,
extract: true,
cssHeaders: {
'x-my-fancy-header': '... some custom header value...',
...
}
})
```

I'm open to suggestions on a more elegant way to address the issue as well as thoughts on naming. I'm happy to open a PR to this effect, but wanted to vet the idea here to gauge consensus first.