keepTyping(curString, curStrPos, numChars) {
    // call before functions if applicable
    if (curStrPos === 0) {
      this.toggleBlinking(false);
      this.options.preStringTyped(this.arrayPos, this);
    }
    // start typing each new char into existing string
    // curString: arg, this.el.html: original text inside element
    curStrPos += numChars;
    const nextString = curString.substring(0, curStrPos);
    this.replaceText(nextString);
    // loop the function
    this.typewrite(curString, curStrPos);
  }
Continue to the next string & begin typing
@param {string} curString the current string in the strings array
@param {number} curStrPos the current position in the curString
@private