Add ui-util formatting & util (#76)

* Add ui-util formatting & util

* Update README

packages/ui-util/src/formatDecimal.ts

@@ -0,0 +1,13 @@
+// Copyright 2017-2019 @polkadot/ui-util authors & contributors
+// This software may be modified and distributed under the terms
+// of the Apache-2.0 license. See the LICENSE file for details.
+
+const NUMBER_REGEX = new RegExp('(\\d+?)(?=(\\d{3})+(?!\\d)|$)', 'g');
+
+export default function formatDecimal (value: string): string {
+  const matched = value.match(NUMBER_REGEX);
+
+  return matched
+    ? matched.join(',')
+    : value;
+}