title: Extend Leaflet Marker with Typescript
body: <p>I'm working on a project with Typescript and Leaflet.</p>
  <p>The documented (JS) way to extend the leaflet marker is like this:</p>
  <pre><code>L.Marker.Foo = L.Marker.extend({...});
  </code></pre>
  <p>However, Typescript complains:</p>
  <pre><code>Property 'Foo' does not exist on type 'typeof Marker'.
  </code></pre>
  <p>How can I change it so there are no compile errors?</p>