Add 'context' BMO menu for switching taskwarrior context

dotfiles/bmo/menu.ini

     item = opera-rh
     item = opera-beta
     consumer = cat
+
+[menu.context]
+
+    generator = echo none; task context list | grep '^[a-z]'
+    select_args = --split ' ' --split-field 1
+    consumer = xargs -r task context