Two-way binding for grouped checkbox inputs

[Rich-Harris]

Similar to #311, except with checkboxes. Ractive equivalent is here. If we adopted the proposal in #311, then the same logic applied to this case would result in...

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

...where selected was an array with the values of all the currently checked checkboxes with name='whatever'.