Add `addclass` helper to live-bind additive classes

* Adding classes within the `class` attribute will cause any
  externally-added classes to be lost after the bound value changes.
  With `addclass`, we instead bind to the attribute/compute explicitly
  and add/remove the class as it changes.
* `addclass` will also unbind its callback if the element is detached
  from the DOM.
Merge pull request #1220 from reciprocity/feature/fix-request-tree

Add `addclass` helper to live-bind additive classes