Enable author to avoid default keyboard bindings for certain elements, like text input elements or code editors.

Implemented [data-keyboard-capture], so elements inside slides that use keyboard events are not captured by impress.

USAGE:
------

```html
<div id="code-demo" class="step slide" data-x="4400" data-y="2200" data-z="800" data-rotate-x="0">
    <textarea id="code-editor" data-keyboard-capture="true"></textarea>
</div>
```

All keyboard events originating on the textarea, or childs of that element (for example if it gets converted to a CodeMirror instance) are ignored by the impress.js keyboard event handlers.