feat(components): add RadioButtonGroup

Closes #25

src/components/RadioButtonGroup/RadioButtonGroup.svelte

@@ -0,0 +1,46 @@
+<script>
+  let className = undefined;
+  export { className as class };
+  export let orientation = 'horizontal';
+  export let labelPosition = 'right';
+  export let defaultSelected = undefined;
+  export let disabled = false;
+  export let style = undefined;
+
+  import { createEventDispatcher, setContext } from 'svelte';
+  import { writable } from 'svelte/store';
+  import { cx } from '../../lib';
+
+  const dispatch = createEventDispatcher();
+  const _class = cx(
+    '--radio-button-group',
+    orientation === 'vertical' && `--radio-button-group--${orientation}`,
+    labelPosition && `--radio-button-group--label-${labelPosition}`,
+    className
+  );
+
+  let selected = writable(defaultSelected);
+
+  setContext('RadioButtonGroup', {
+    selected,
+    add: ({ checked, value }) => {
+      if (checked) {
+        selected.set(value);
+      }
+    },
+    update: value => {
+      selected.set(value);
+    }
+  });
+
+  $: {
+    defaultSelected = $selected;
+    dispatch('change', $selected);
+  }
+</script>
+
+<div on:click on:mouseover on:mouseenter on:mouseleave class={cx('--form-item')} {style}>
+  <div class={_class} {disabled}>
+    <slot />
+  </div>
+</div>