Support two-way binding for checkboxes in LinkedValueMixin via 'checkedLink' property

This is an attempt to fix #475.
As per petehunt's suggestion on the issue, I added a new `checkedLink` property for `input` components, that behaves like `valueLink`, but reads/updates the `checked` status instead of `value`. I chose not to use `valueLink`/`value`, as that property already has a [clear function in the HTML spec](http://www.w3.org/TR/html401/interact/forms.html#adef-value-INPUT) for inputs of type checkbox.