Move wttr to separate nag stream

Unfortunately wttr often gets flaky after period of not using it
(apparently some cache rot).

dotfiles/config/bmo/nag.ini

 [nag.stream]
-    main = stconflict wttr timenow
+    main = stconflict timenow
+    wttr = wttr
 
 [nag.def.timenow]
     command = date +%H:%M