[REQ] Impl attribute parsing and rendering

[mhatzl]

Attribute syntax changed from "JSON-like" to "CSS-like".
Therefore, parsing must be reworked.

Besides CSS properties, attributes also allow most HTML attributes to be set.
Unless other conflicts between CSS props and HTML attributes arise, all HTML attributes besides style, height, and width should be ok to use.

A restriction for HTML attributes is that they must not be allowed inside CSS at-rules, since Unimarkup cannot predict at compile time what at-rules should be chosen.

[mhatzl]

The id attribute is also not allowed in nested context, because an id must be unique per element.
Other HTML attributes in nested context should be allowed, but the behavior may differ between attributes. e.g. the class attribute should merge set classes, while disabled is taken only from the most inner one.