title: Extension fields in Kotlin
body: <p>It's easy to write extension methods in Kotlin:</p>
  
  <pre><code>class A { }
  class B {
      fun A.newFunction() { ... }
  }
  </code></pre>
  
  <p>But is there some way to create extension variable? Like:</p>
  
  <pre><code>class B {
      var A.someCounter: Int = 0
  }
  </code></pre>