folderAdd = function(name, createFolders) {
    createFolders = (typeof createFolders !== 'undefined') ? createFolders : defaults.createFolders;

    name = forceTrailingSlash(name);

    // Does this folder already exist?
    if (!this.files[name]) {
        fileAdd.call(this, name, null, {
            dir: true,
            createFolders: createFolders
        });
    }
    return this.files[name];
}
Add a (sub) folder in the current folder.
@private
@param {string} name the folder's name
@param {boolean=} [createFolders] If true, automatically create sub
 folders. Defaults to false.
@return {Object} the new folder.