Allow to specify content for entity items

[igorlesnenko]

var markdownString = draftToMarkdown(rawObject, {
  entityItems: {
    mention: {
      open: function (entity) {
        return '<span class="mention-item" data-user-id="' + entity.data.id + '">';
      },

      close: function (entity) {
        return '</span>';
      }
    }
  }
});

We have a way to specify open and close, but no content. Suggest to add:

text: function (entity) {
}

not sure about function name

[igorlesnenko]

The use case is to display a mention like @nickname as Firstname Lastname in draftjs. But as @nickname in markdown.

[RyanEdwardHall]

It would be nice to display Firstname Lastname but serialize as @{userID}