src/PopupTools.js
function optionsResolveCentered(options) {
    var result = options;
    var width = window.outerWidth - options.width;
    var height = window.outerHeight - options.height;

    if (options.centered) {
        result.left = options.left || Math.round(window.screenX + width / 2);
        result.top = options.top || Math.round(window.screenY + height / 2.5);
        delete result.centered;
    }
    return result;
}
function optionsResolveCentered(options) {
    var result = options;
    var width = window.outerWidth - options.width;
    var height = window.outerHeight - options.height;

    if (options.centered) {
        result.left = options.left || Math.round(window.screenX + width / 2);
        result.top = options.top || Math.round(window.screenY + height / 2.5);
        delete result.centered;
    }
    return result;
}
Convert "centered: true" key into concrete left and top arguments
Both can be overwritten

@param  {Object} options
@return {Object}