Add a SFW mode

[CodeCanna]

I would like to add a way to make it easy to toggle between the ratings. There are 3 on e621, explicit, questionable, and safe.

Right now to pick a rating the user must enter a keyword for the rating safe, explicit, and questionable. This will set the bot to that rating but only for that query. This would be a way to lock the rating to a specific rating so the user only need send their query. If they search for something explicit while the rating is safe, they will simply get no results.