fix to accept comments for json literals

Related with #378
By this patch, v1config accepts comments for json objects with newlines, ex:
```
<section>
  attr1 {
    "key1": 1, # This is 1.
    "key2": 2, # This is 2.
    "key3": 3  # Tailing commas are prohibited on JSON
  }
</section>
```