title: typescript add properties to Vue library definitions
body: <p>I use vue which includes types. However I want to use some properties that are added by certain plugins.</p>
  
  <p>e.g. </p>
  
  <pre><code>Vue.$ga
  Vue.$router
  </code></pre>
  
  <p>Typescript complains that Vue does not have those properties. Can I somehow add those globally to the Vue definition?</p>