Custom authentication parameter

Implementing the authentication parameter to be configurable. (Instead of hardcoded `email`)

Example:

    SimpleTokenAuthentication.configure do |config|
      config.auth_parameter_name = { user: 'your_parameter_name' }
    end

#68 
I think for my case that is using a multi-word field like phone_number this would result in "X-User-Phone_number". Maybe we would want this to be "X-User-Phone-Number"?