Add option to update a macro's icon with the...

... current pet's special ability
-- Add slash command (/psab_update, /psabu) to manually trigger an icon
update
- Change EXCEPTIONS table to OVERRIDES in generate script
-- Change output for each family to show whether the selected ability is
an override
- Add overrides for Cat, Shale Spider and Silithid as per Tybudd's
recommendations
-- Re-generate abilities_enUS.lua with these overrides