'headers' section of config that follows routing rules

Adds support for a section of the configuration:

```json
{
  'headers': {
    '/test1': {
      'Content-Type': 'mime/type'
    },
    '/test3': {
      'Access-Control-Allow-Origin': 'https://www.example.net'
    },
    '/api/**': {
      'Access-Control-Allow-Origin': '*'
    }
  }
}
```

Sets headers when a request matches the route (same routing rules as custom routes)