title: Is there joinWithSeparator for attributed strings
body: <p>Array of strings can be joined together with specific separator using joinWithSeparator method. </p>
  
  <pre><code>let st = [ "apple", "pie", "potato" ]
  st.joinWithSeparator(", ")
  </code></pre>
  
  <p>As a result we will have "apple, pie, potato". </p>
  
  <p>What if I have attributed strings inside my array? Is there easy way to combine them into one big attributed string? </p>