### on

Adds an event listener to an element with the ability to use event delegation.

Use `EventTarget.addEventListener()` to add an event listener to an element. If there is a
`target` property supplied to the options object, ensure the event target matches the
target specified and then invoke the callback by supplying the correct `this` context.
Use `options` to pass options to `addEventListener` or omit it to use bubbling by default.
In order to use this function with `off`, the reference to the custom delegator function
is returned if the `target` option is specified.

```js
const on = (el, evt, fn, opts = {}) => {
  const delegatorFn = e => e.target.matches(opts.target) && fn.call(e.target, e);
  el.addEventListener(evt, opts.target ? delegatorFn : fn, opts.options || false);
  if (opts.target) return delegatorFn;
};
```

```js
const fn = () => console.log('!');
on(document.body, 'click', fn); // logs '!' upon clicking the body
on(document.body, 'click', fn, { target: 'p' }); // logs '!' upon clicking a `p` element child of the body
on(document.body, 'click', fn, { options: true }); // use capturing instead of bubbling
```