feat(components): add RadioButtonGroup

Closes #25

src/components/RadioButtonGroup/RadioButtonGroup.Story.svelte

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+<script>
+  import Layout from '../../internal/ui/Layout.svelte';
+  import RadioButtonGroup from './RadioButtonGroup.svelte';
+  import RadioButton from '../RadioButton';
+  import { FormGroup } from '../Form';
+</script>
+
+<Layout>
+  <FormGroup legendText="Radio Button heading">
+    <RadioButtonGroup {...$$props.group} defaultSelected="default-selected" legend="Group Legend">
+      <RadioButton {...$$props.radio} value="standard" id="radio-1" />
+      <RadioButton {...$$props.radio} value="default-selected" id="radio-2" />
+      <RadioButton {...$$props.radio} value="disabled" id="radio-3" />
+    </RadioButtonGroup>
+  </FormGroup>
+</Layout>

src/components/RadioButtonGroup/RadioButtonGroup.stories.js

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+import { withKnobs, text, select, boolean } from '@storybook/addon-knobs';
+import Component from './RadioButtonGroup.Story.svelte';
+
+export default { title: 'RadioButtonGroup', decorators: [withKnobs] };
+
+const values = {
+  standard: 'standard',
+  'default-selected': 'default-selected',
+  disabled: 'disabled'
+};
+
+const orientations = {
+  'Horizontal (horizontal)': 'horizontal',
+  'Vertical (vertical)': 'vertical'
+};
+
+const labelPositions = {
+  'Left (left)': 'left',
+  'Right (right)': 'right'
+};
+
+export const Default = () => ({
+  Component,
+  props: {
+    group: {
+      name: text('The form control name (name in <RadioButtonGroup>)', 'radio-button-group'),
+      valueSelected: select(
+        'Value of the selected button (valueSelected in <RadioButtonGroup>)',
+        values,
+        'default-selected'
+      ),
+      orientation: select('Radio button orientation (orientation)', orientations, 'horizontal'),
+      labelPosition: select('Label position (labelPosition)', labelPositions, 'right')
+    },
+    radio: {
+      disabled: boolean('Disabled (disabled in <RadioButton>)', false),
+      labelText: text('Label text (labelText in <RadioButton>)', 'Radio button label')
+    }
+  }
+});

src/components/RadioButtonGroup/RadioButtonGroup.svelte

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+<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>

src/components/RadioButtonGroup/index.js

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+import RadioButtonGroup from './RadioButtonGroup.svelte';
+
+export default RadioButtonGroup;