Support data attributes on HTML tags

[flyx]

HTML allows the usage of arbitrary data-* attributes on any tag. emerald should support that. Possible implementation:

emerald:

p(data={str="foo", num=5}): "content"

output:

<p data-str="foo" data-num="5">content</p>

Open questions:

• Is the value required to be a table constructor literal?
• If not, can it take a Table, a seq, something else as value?