Also return the number of affected entities

Closes #293. Doesn't really solve anything there aside from exposing the
number, but sadly there's not really anything obvious I can do on my end
- the command API just doesn't expose anything else.

src/main/java/dan200/computercraft/shared/computer/apis/CommandAPI.java

@@ -83,7 +83,7 @@ private Object[] doCommand( String command )
         {
             sender.clearOutput();
             int result = commandManager.executeCommand( sender, command );
-            return new Object[] { result > 0, sender.copyOutput() };
+            return new Object[] { result > 0, sender.copyOutput(), result };
         }
         catch( Throwable t )
         {