Add dataset as property.

When virtualizing nodes data attributes were skipped. There is code for virtualizing them in _index.js_ starting at line 95, but this depends on `"dataset"` being present in the list of properties.

I noticed the comment `// However, I'm not sure if this is compatible with h()`, so maybe that's why it's not in the properties list. However, using `h()` to create data properties has been working fine for me using `"virtual-hyperscript": "4.4.0"`.