hear(keywords, callback) {
    keywords = Array.isArray(keywords) ? keywords : [keywords];
    keywords.forEach(keyword => this._hearMap.push({ keyword, callback }));
    return this;
  }
A convinient method to subscribe to message events containing specific keywords. The keyword param can be a string, a regex or an array of both strings and regexs that will be tested against the received message. If the bot receives a message that matches any of the keywords, it will execute the specified callback. String keywords are case-insensitive, but regular expressions are not case-insensitive by default, if you want them to be, specify the i flag.
@param {String|Regex|Array} keywords
@param {Function} callback