Open image link in new tab

The screenshot has "play button" which tricks people to start playback.
In most cases they don't want to leave the current page, so opening
asciicast link in a new tab seems like the best solution.

app/presenters/asciicast_page_presenter.rb

@@ -62,7 +62,7 @@ class AsciicastPagePresenter
     img_src = routes.asciicast_url(asciicast, format: :png)
     url = routes.asciicast_url(asciicast)
     width = %{width="#{asciicast.image_width}"} if asciicast.image_width
-    %(<a href="#{url}"><img src="#{img_src}" #{width}/></a>)
+    %(<a href="#{url}" target="_blank"><img src="#{img_src}" #{width}/></a>)
   end
 
   def embed_markdown_link