code: function indexOfWidth(stringWidth, str, width) {
  	if (stringWidth === false)
  		return width;
  	for (var i = 0; i <= str.length; i++) {
  		if (getWidth(stringWidth, str.substr(0, i)) > width)
  			return i - 1;
  	}
  	return str.length;
  }
docs: calculate the position that the prefix of string is a specific width
  @params {string} str - string to calculate
  @params {number} width - the width of target string
  @params {boolean} stringWidth - calculate width by wcwidth or String.length