title: ReactRouter "on change" event?
body: <p>I want to install an analytics handler on my single page app, and would like to capture every route change under the main route. I recognize that ReactRouter offers an <code>onEnter</code> route event; however, this would require me to separately install the handler on each route. I could create a subclass of Route that automatically has <code>onEnter</code> populated, but then I would not be able to override <code>onEnter</code> and still keep the analytics tracking handler.</p>
  
  <p>What would be the best way to listen for any change within a given root Route?</p>