title: UIActivity Subclass Mail
body: <p>I am using <code>UIActivityViewController</code> to share data from my app. I know how I can create a custom <code>UIActivity</code> subclass and include that on my share sheet.</p>
  
  <p>However, is it possible to customise the system activities? For example, I want to send some data via an email. Can I customise the system one to take my data, subject etc?</p>
  
  <p>I don't want to exclude <code>UIActivityTypeMail</code> and then just create a custom one, I want my data etc going through the system Mail activity.</p>