initial idea for tags dsl

@ianstormtaylor initial idea to go along with https://github.com/segmentio/analytics.js-integration-tester/pull/25
these overloads could be written to get out of the way like so:

```js
if ('function' == typeof name) fn = name, locals = null, name = null;
if ('function' == typeof locals) fn = locals, locals = null;
name = name || 'main';
locals = locals || {};
```

which also guarantees one code path for everything, instead of setting the defaults in multiple places