Add 'input' binding for real-time updates from text fields

The `value` binding documentation includes examples of `valueUpdate` options (`keyup`, `keypress`, and `afterkeydown`) and recommends `afterkeydown` as the best choice to keep the view model updated in real-time. But in modern browsers, `input` is actually the best choice since it fires for changes from mouse events (such as from the context menu or drag-and-drop).

I propose that we present the `input` event as the preferred option for real-time updates. In browsers that don't support `input`, we can simulate it using `afterkeydown` and/or `propertychange`.

In the meantime, we should add `input` as a suggested option in the documentation.