Make the onPreview() callback asynchronous.

[nud]

This makes it possible to perform asynchronous operations while in the
onPreview() callback, for instance using an AJAX request:

$("#form_content").markdown({
  onPreview: function(editor, callback) {
    jQuery.ajax({
      type: "POST",
      url: "/render-markdown",
      data: { content: editor.$textarea.val() },
      success: callback
    });
  }
});

A default implementation of onPreview() is provided, using an updated
parseContent() method which doesn't call the onPreview() callback
anymore.

This effectively adds a parameter to the onPreview() callback, but
backward compatibility is retained by checking the onPreview() callback
return value and using it if it exists.

[toopay]

Hey @nud Can you give me an edge-case whereas we need the onPreview loaded asynchronuously?

[nud]

Well the case is in the pull request: you need an async callback if you want to do the markdown rendering server-side, which is needed if you want to do fancy stuff in the markdown rendering.

Doing a synchronous ajax call as two downsides:

1. it is deprecated
2. it can freeze the browser if the server takes some time to respond.