Add /tracemask. (#474)

Allows setting a mask used for block traces. This allows brush tools to
pass through various materials, such as water (e.g. `/tracemask #solid`
or `/tracemask !air,water`) before starting to build.
By default, a null mask is equivalent to #existing (original behavior).

https://gfycat.com/ImmaculateFrayedCockatiel

worldedit-core/src/main/java/com/sk89q/worldedit/session/request/RequestExtent.java

@@ -45,7 +45,8 @@ public class RequestExtent implements Extent {
         if (request == null || !request.isValid()) {
             request = Request.request();
         }
-        return request.getEditSession();
+        final EditSession editSession = request.getEditSession();
+        return editSession == null ? request.getWorld() : editSession;
     }
 
     @Override