Binds the given expression to the value of `<option>` or `input[radio]`,
so that when the element is selected, the `ngModel` of that element is set to
the bound value.
`ngValue` is useful when dynamically generating lists of radio buttons using
`ngRepeat`, as shown below.
Likewise, `ngValue` can be used to generate `<option>` elements for
the `select` element. In that case however, only strings are supported
for the `value `attribute, so the resulting `ngModel` will always be a string.
Support for `select` models with non-string values is available via `ngOptions`.