Update to use rollup.js

Thanks @ljmerza!

rollup.config.js

@@ -0,0 +1,14 @@
+import resolve from 'rollup-plugin-node-resolve';
+import typescript  from 'rollup-plugin-typescript2';
+
+export default {
+    input: 'src/index.ts',
+    output: {
+        file: 'boilerplate-card.js',
+        format: 'es'
+    },
+    plugins: [
+        resolve(),
+        typescript ()
+    ]
+};