support custom events to trigger update

This PR provides a way to use peek-performance_bar with standard Ajax requests that are not carried out via Pjax or Turbolinks, simply by supporting custom `peek:start` and `peek:end` events.

For example, you could do something like this to update the performance bar for all Ajax requests:
```javascript
$(document).on('ajax:send', function() { $(this).trigger('peek:start') })
$(document).on('ajax:complete', function() { $(this).trigger('peek:end') })
```