title: To set exception recipients email in environment variable
body: <p>As shown in code below, recipient's mails are hard-coded, how can I set them in environment variable</p>
  
  <pre><code>  Example::Application.config.middleware.use ExceptionNotification::Rack,
      :email =&gt; {
        email_prefix: "[Error] ",
        sender_address: %{"Exception Notifier" &lt;exception@example.com&gt;},
        exception_recipients: %w{example@example.com example2@example.com}
    }
  </code></pre>
  
  <p>I've tried <code>exception_recipients: ENV['mail']</code> <code>%w{ENV['mail']}</code> <code>"%w{#{ENV['mail']}}"</code>, but none works, it gives syntax error</p>