Allow CKEDITOR.template to use a callback as a source of a template

[mlewand]

Type of report

Feature request

Provide description of the new feature

Based on PR #2799 request:

The main reason is I want to use it in configuration of 'mentions' plugin
Some of the users I want to mention does not have avatars, in this case I want to display their initials instead of unreachable image.

var config = {
mentions: [
    {
        feed: dataFeed,
        marker: '@',
        itemTemplate: function(data) {
            return '<li data-id="{id}" class="autocomplete-item">' +
                    (data.imageUrl ? '<img class="avatar" src="{imageUrl}"/>' : '<span class="initials">{initials}</span>') +
                    '<span>{label}</span>' +
                '</li>'
        },
        outputTemplate: '[[{type}|{id}|{label}]]<span>&nbsp;</span>',
        minChars: 1
    }
]
}

[mlewand]

Closed with #2822.