Allow for catch-all whitelist on a tag name

Setting a tag to 'true' in the sanitizer whitelist object will pass any attribute
through for that node. So, for example:

```js
var janitor = new HTMLJanitor({
	tags: {
		div: {'id': true, 'class': true}, a: {'href': true, 'target': '_blank'}, p: {}, span: true
	}
});
```

Will allow no attributes for `p` as per usual, but all attributes regardless of their value for `span`.