support boolean values for boolean attributes

Even though the correct HTML5 thing to do is to use valueless boolean attributes or to not add them at all (i.e. `` versus ``), this becomes a bit awkward when generating them programmatically. In particular, if one wants to add observables to Hyperscript to add interactivity (see https://github.com/SimonDanisch/JSServe.jl/pull/35 for instance), things become a bit awkward.

From what I understand, HTML5 actually disallows using `"true"` or `"false"` as values for boolean attributes (see [here](https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes#Boolean_Attributes)). Could it be possible to support passing `checked = false` to mean that the attribute is actually not set, and `checked = true` to set it?

I confess I'm not sure what the attribute should be set to with `checked = true`, but I imagine anything that conforms to HTML5 would be fine (empty string, or name of the attribute, or maybe even valueless).