Two-way binding for grouped radio inputs

[Rich-Harris]

Via #10 / #309. In Ractive, you could do this:

<label>
  <input type="radio" value="Alpha" name='{{selected}}' />Alpha
</label>

<label>
  <input type="radio" value="Beta" name='{{selected}}' />Beta
</label>

<label>
  <input type="radio" value="Gamma" name='{{selected}}' />Gamma
</label>

<p>selected: {{selected}}</p>

The name='{{selected}}' sort of makes sense, in that a shared name identifies the radios as belonging to the same group, but only sort of, since you're not actually binding to the name.

A possible alternative — keen to hear what people think:

<label>
  <input type="radio" value="Alpha" name='whatever' bind:group='selected' />Alpha
</label>

In other words, selected would take the value of whichever radio in the group of those with name='whatever' was checked.

[Conduitry]

How is bind:xxx currently handled for non-components (i.e. for regular elements)? The docs mention bind:[attribute] but does it even make sense currently to talk about anything other than bind:value? I was initially worried about bind:group being a special case just for the name group but then realized that probably every different bind:xxx is going to be a special case on regular elements.

[Rich-Harris]

There's also bind:checked. Should probably do some validation to make sure that no-one's doing daft stuff like <input bind:potato> or <div bind:value> — could be a compile-time error