Example notebooks are broken

Right now, our _html_repr_ is not handling a couple of our features:

[x] Boolean key-value pairs (all values are treated as strings meaning you cannot define <video controls><source src='…'></video> as opposed to <video ><source src='…'></video>)
[x] DOM style css keys don't work in style attribute blocks (inline style info needs to be defined with css-style names)
[ ] Updating existing display objects does not work since we changed VDOM objects to be immutable; this breaks the last examples in exploring-elements.ipynb
- But note that we do have updated progressbars in custom-progress-bars.ipynb

I've got a PR to fix the first two but the last one is more of a design decision on how we want to support updated displays of vdom objects.

@rgbkrk @yuvipanda

nteract / vdom