Pass HTTP header "Authorization"

To connect to my remote API I have to provide a Token using the "Authorization" HTTP header:
```
curl -H 'Authorization: <my-secret-token>' https://api.example.com/v0/books
```

How can I provide such HTTP header to expvarmon?

Is this issue related to https://github.com/divan/expvarmon/issues/23?