Test Onionoo cannot be invoked without 'new'

test/unit.js

@@ -4,3 +4,8 @@ import Onionoo from '../'
 test('Onionoo is a function', t => {
   t.is(typeof Onionoo, 'function')
 })
+
+test('Onionoo cannot be invoked without \'new\'', t => {
+  t.throws(() => Onionoo())
+  t.notThrows(() => new Onionoo())
+})