Add dataset to default snabbdom modules for html

[shesek]

Added to snabbdom in snabbdom/snabbdom-to-html@3f37386

See https://github.com/snabbdom/snabbdom#the-dataset-module

[staltz]

Thanks for the PR, looks good! Could you rebase it to use the commit message convention? (use npm run commit). See the instructions we have in all pull requests: https://github.com/cyclejs/cyclejs/blob/master/.github/PULL_REQUEST_TEMPLATE.md