Add iteration index to rv-each context

This seems like a really simple thing, so I've probably drastically misread something :) Anyway, I want to be able to reference the iteration index from inside the each context. For example:

```html
<select rv-value="selectedItem">
  <option rv-each-item="items" rv-value="index">{ item }</option>
</select>
```

Results in:
```html
<select value="1">
  <option value="0">Foo</option>
  <option value="1">Bar</option>
</select>
```

Sorry if this has been brought up before or if there's some way easier way to do this and I'm just being stupid. At least it's a simple change!