Initial improvement in sp_whoisactive (#1249)

* improve sp_whoisactive extension

* formatting

* use a grouping prefix for extension commands

samples/sp_whoIsActive/src/sql/blockingSessions.sql

@@ -0,0 +1,4 @@
+SELECT blocking_session_id
+FROM sys.dm_os_waiting_tasks
+WHERE 
+    blocking_session_id IS NOT NULL