feat(component): add ProgressIndicator

Closes #24

src/components/ProgressIndicator/ProgressIndicator.svelte

@@ -0,0 +1,39 @@
+<script>
+  let className = undefined;
+  export { className as class };
+  export let currentIndex = 0;
+  export let vertical = false;
+  export let style = undefined;
+
+  import { createEventDispatcher, setContext } from 'svelte';
+  import { writable, derived } from 'svelte/store';
+  import { cx } from '../../lib';
+
+  const dispatch = createEventDispatcher();
+  const _class = cx('--progress', vertical && '--progress--vertical', className);
+  let steps = writable([]);
+  let stepsById = derived(steps, $steps => $steps.reduce((a, c) => ({ ...a, [c.id]: c }), {}));
+
+  setContext('ProgressIndicator', {
+    steps,
+    stepsById,
+    add: step => {
+      steps.update(_ => [
+        ..._,
+        {
+          ...step,
+          index: _.length,
+          current: _.length === currentIndex,
+          complete: _.length <= currentIndex
+        }
+      ]);
+    },
+    change: index => {
+      dispatch('change', index);
+    }
+  });
+</script>
+
+<ul on:click on:mouseover on:mouseenter on:mouseleave class={_class} {style}>
+  <slot />
+</ul>