add attr-foo support, related to #6

[ahdinosaur]

it's purely sugar, as now i realize attributeHook is included in virtual-hyperscript.

[Raynos]

This might not be the best way to do attributes.

I like the syntax, but I think hooks are too heavy.

cc @Matt-Esch does props.attributes.x = bar work ?

[ahdinosaur]

looks like it: Matt-Esch/vdom#11 and https://github.com/Matt-Esch/virtual-dom/blob/master/test/attributes.js.

mercury is not using that version of vdom though, so this is the only way i'm aware of that works right now with the rest of mercury. however, i'm fine using attributeHook for now and once we can pass in attributes as a property, this sugar seems unnecessary.

[Raynos]

the sugar is nice for consistency. I prefer 'attr-foo-bar': x over attributes: { 'foo-bar': x }