New: Complex Keys (cls, rel, textContent, etc.)

[raquo]

This PR provides a way for consuming libraries to define custom interfaces for some composite or otherwise special keys.

The attributes I chose are: cls, className, rel, dataAttr, styleAttr, textContent.

I was thinking of also adding role attribute to the list – it's also a composite attribute (space-separated strings), but I've never used it and I'm not sure what kind of usage patterns it has. Does it even make sense to have special APIs for it...

Fixes #32.

TODO

• Change v0.9 date from Aug to Sep

[cornerman]

Not entirely sure whether cls and className really need different types, but otherwise looks good to me.

[raquo]

Agreed, I think I'll change className to be an alias for cls. Can't really imagine libraries wanting those exact names but different implementations.

I think I'll add the role attribute in there after all, for the sake of consistency.

What do you think about removing textContent from this trait (and SDT) for reasons I mentioned in #41?

In contrast to these fringe properties, ComplexHtmlKeys is supposed to have keys that every library would want to implement, but perhaps differently. TBH I'm still second guessing even adding textContent to it, I might actually remove it from there because I'm not sure how universally supported its behaviour – modifying the tree of elements – would be in consuming libraries.

[cornerman]

Yep, then better remove textContent as it falls into the same class as innerHTML