Adds options.headers being set on any request to files within the content base

Adds support for specifying `options.headers` which are then set on any request to a file within the content base not just the bundles. The pull requests relates to issue #69. 

A `webpack.config.js` such as 
```javascript
...
devServer: {
   headers: { "Access-Control-Allow-Origin": "*" }
}
...
```
would set the `Access-Control-Allow-Origin` entry on e.g. the `index.html` within the content base.