can.ui.Selectable API

## Purpose

Provide keyboard support
## Options
- selects - what types of elements we can select
- selectClassName
- activeClassName
- multiActive
- cache
- outsideDeactivate
## Events

typically get a model instance (probably not anymore)
- select / deselect
- activate / deactivate
## Methods
- activated
- selected
## Theming
## Examples
- Use with a grid
- A navigation widget
## Notes

I'd like to make another one of these that can go "into" and "out" of widgets allowing you to traverse the page.

Start by reviewing the proposed can.ui.Selectable options, events, methods, and keyboard-support purpose in this issue. Define the API and its behavior for selection, activation, deactivation, caching, and traversal before implementation. Done should include the described grid and navigation-widget examples.

Written by the indexing model from the issue text.