title: Custom message in devise invitable email
body: <p>How do you pass a custom message to the devise invitable email? I want the inviter to include a message to the invitee, like "Hey check out this site".</p>
  
  <p>I tried both including it in the attributes and setting an instance variable in the block, neither seem to be accessible from the email.</p>
  
  <pre><code>  user = User.invite!(:email =&gt; share.to_user_email, :message =&gt; "hey check this out") do 
      @message = "hey it's me!"
    end
  </code></pre>