Request feature: runtime composition of ko 3.2 component

Right now, ko 3.2 component supports composition directly in component's template. This kind of composition only happens in definition time.

Would suggest to support runtime composition, so we can define kind of "container" component, and make tags more expressive.

The usage is something like this:

    <ko-toolbar params="..."> <!-- this is a container component -->
      <!-- compose buttons at runtime -->
      <ko-toolbar-button params="..."></ko-toolbar-button>
      <ko-toolbar-button params="..."></ko-toolbar-button>
    </ko-toolbar>

FYI, this feature is available on [Google Polymer](http://www.polymer-project.org) through `<content>` tag,