Add element.dataset API

[karlseguin]

Uses the State to store the dataset, but, on first load, loads the data attributes from the DOM.

[krichprollsch]

LGTM, but I'm wondering about attributes synchronization:

• should be data-x attributes accessible from attributes list?
• what about generating the html dump? if we don't sync with attr, we could loop hover the dataset to build the valid final html dump.

It's similar to our input problem I suppose. That's something we can work on in a future PR.

[karlseguin]

LGTM, but I'm wondering about attributes synchronization:

You're right. I did a follow up commit. DataSet now just wraps the *parser.Element and uses elementGetAttribute, elementSetAttribute, elementRemoveAttribute.

I tested in browser, and this is more accurate to how it should work - changes to the dataset are reflected in the attributes and vice-versa.