message: substring method
diff: export default class StringBuilder {
  +
  public constructor(private value: string = '') {
  }
  +
  public append(s: string | number): StringBuilder {
  if (typeof s === 'string') {
  this.value += s.toString();
  @@ -27,6 +29,10 @@ export default class StringBuilder {
  this.value = this.value.substr(0, n) + c + this.value.substr(n + 1);
  }
  + public substring(start: int, end: int): string {
  + return this.value.substring(start, end);
  + }
  +
  public toString(): string {
  return this.value;
  }
  