Add RadioGroupViewElement impl

First off, I'm not sold on the class name for this one. Second, #getCurrentlySelectedOption I have returning either simply a Radio object or null. Thinking about that while I type this, it probably shouldn't return null but I also don't feel it should return Optional<Radio> -- IIRC, say you have a group of radio buttons: at least one is always selected at some point as opposed to something like a group of checkboxes where all, a few, or none can be checked. Hmm, I dunno. Let me know what you think of that method. Thanks!
In the meantime I recommend setting up a test html page where you can verify it works on these edge cases.

Take this snippet:
```html
<input id='1' type="radio" name="foo">
<input id='2' type="radio" name="foo" checked>
<input id='3' type="radio" name="foo">

<form>
    <input id='4' type="radio" name="foo">
    <div>
        <input id='5' type="radio" name="foo">
    </div>
    <form>
        <input id='6' type="radio" name="foo">
    </form>
</form>

<div>
    <input id='7' type="radio" name="foo">
</div>
```

There are 3 radio groups here, despite all having the same name:
Group 1 has ids 1, 2, 3, and 7
Group 2 has ids 4 and 5
Group 3 has ids 6

You should be able to instantiate a radio group that matches each of these three, and getOptions should only return the elements corresponding with those ids for each group.