feat(tile): complete RadioTile

Closes #34

- Add Tile to list of supported components
- Make RadioTile composable as a "child" component

src/components/Tile/TileGroup.svelte

@@ -1,12 +1,50 @@
 <script>
   let className = undefined;
   export { className as class };
+  export let defaultSelected = { value: undefined };
   export let disabled = false;
   export let legend = '';
   export let style = undefined;
 
+  import { createEventDispatcher, setContext } from 'svelte';
+  import { writable } from 'svelte/store';
   import { cx } from '../../lib';
 
+  const dispatch = createEventDispatcher();
+  let tiles = [];
+  let selected = writable(defaultSelected);
+
+  setContext('TileGroup', {
+    selected,
+    addTile: tile => {
+      tiles = [...tiles, tile];
+    },
+    updateSelected: tile => {
+      selected.set(tile);
+    }
+  });
+
+  $: {
+    const checkedTiles = tiles.filter(tile => tile.checked);
+
+    if (checkedTiles.length > 1) {
+      console.warn('Multiple RadioTiles cannot be checked.');
+
+      if (defaultSelected.value) {
+        console.warn('Using `defaultSelected`:', defaultSelected);
+      } else {
+        console.warn('Using `RadioTile`:', checkedTiles[0]);
+        selected.set(checkedTiles[0]);
+      }
+    } else if (checkedTiles.length === 1) {
+      selected.set(checkedTiles[0]);
+      tiles = [];
+    }
+
+    defaultSelected = $selected;
+    dispatch('select', $selected);
+  }
+
   const _class = cx('--tile-group', className);
 </script>