addWatcher(paths, events, callback) {
        if (!Array.isArray(events)) {
            events = [events];
        }
        let watcher = chokidar.watch(paths, {ignoreInitial: true});
        events.forEach(event => {
            watcher.on(event, callback);
        });
        this.watchers.push(watcher);
    }
set chokidar watchers, following directories and files will be watched:
/pages, /config, /core/index.html.tmpl

@param {string|Array.<string>} paths paths
@param {string|Array.<string>} events events
@param {Function} callback callback