Add ui-util formatting & util (#76)

* Add ui-util formatting & util

* Update README

packages/ui-util/src/formatDecimal.spec.ts

@@ -0,0 +1,19 @@
+// 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.
+
+import formatDecimal from './formatDecimal';
+
+describe('formatDecimal', () => {
+  it('formats decimals in number groupings', () => {
+    expect(formatDecimal('12345')).toEqual('12,345');
+  });
+
+  it('formats decimal-only in number groupings', () => {
+    expect(formatDecimal('test6789')).toEqual('6,789');
+  });
+
+  it('returns input for non-decimal', () => {
+    expect(formatDecimal('test')).toEqual('test');
+  });
+});