Draft support for lazy-widget-ids

It works like this. You have a widget whose *data-id* is dynamic, i.e., it
depends on something else, like it's composed of some object's slug. So you
use the following syntax:

    <div data-id="bind:blog.slug" [...]>

Once blog.slug is loaded, it'll be bound to *data-id* and the widget will be
registered into Dashing and will receive any incoming data.

Since I couldn't find a way to observe an attribute change from within the
view I had to hack something around. I hope you enjoy it.