add a 512k thread stacksize

I am getting the following error:

```
C:\workspace\temp>html5validator --show-warnings --root _site
Found files to validate: 37
StackOverflowError while evaluating HTML schema.
The checker requires a java thread stack size of at least 512k.
Consider invoking java with the -Xss option. For example:

  java -Xss512k -jar ~/vnu.jar FILE.html
```

adding the minimum thread stack size to the command resolves the problem.