feat(component): add NumberInput

Closes #21

src/components/NumberInput/NumberInput.stories.js

@@ -0,0 +1,32 @@
+import { withKnobs, text, boolean, number } from '@storybook/addon-knobs';
+import Component from './NumberInput.Story.svelte';
+
+export default { title: 'NumberInput', decorators: [withKnobs] };
+
+export const Default = () => ({
+  Component,
+  props: {
+    id: 'number-input',
+    label: text('Label (label)', 'Number Input label'),
+    hideLabel: boolean('Hide label (hideLabel)', false),
+    min: number('Minimum value (min)', 0),
+    max: number('Maximum value (max)', 100),
+    value: number('Value (value)', 50),
+    step: number('Step of up/down arrow (step)', 10),
+    disabled: boolean('Disabled (disabled)', false),
+    readonly: boolean('Read only (readonly)', false),
+    invalid: boolean('Show form validation UI (invalid)', false),
+    isMobile: boolean('Mobile variant', false),
+    invalidText: text('Form validation UI content (invalidText)', 'Number is not valid'),
+    helperText: text('Helper text (helperText)', 'Optional helper text.'),
+    light: boolean('Light variant (light)', false)
+  }
+});
+
+export const Skeleton = () => ({
+  Component,
+  props: {
+    story: 'skeleton',
+    hideLabel: boolean('Hide label (hideLabel)', false)
+  }
+});