Allows clean async preview implementation with minimal changes to the code base.

[ghord]

Sample async implementation for preview event:

$("#text-editor").markdown({
    onPreview: function (e, previewContainer) {
        $.ajax({ method: "POST", url: "/news/preview", data: { text: e.getContent() } })
            .then(function (result) {
                previewContainer.html(result);
            })

            //return placeholder string. It will be replaced after successfull ajax call
            return "Loading preview...";
   }
})

[lodev09]

thanks @ghord