'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)