Allow whitespace in handlebars closing sections (fixes #983)

Since closing sections capture everything inside them, this was failing with handlebars syntax:

```js
if ( child.t === types.CLOSING ) {
  if ( expectedClose && child.r !== expectedClose ) { // "each" !== "each "
    parser.error( 'Expected {{/' + expectedClose + '}}' );
  }
  break;
}
```

It was OK with standard mustache syntax, because `expectedClose` is `undefined` in that case. Will wait for @Rich-Harris before merging this.