Add config option to change toot visibility

Some instances allow bots only if their toots are unlisted,
in order to avoid flooding the public timeline (makes sense).

README.md

@@ -40,6 +40,8 @@ Alternatively you can donate cryptocurrencies:
         instance_url=https://mastodon.social
         user_credentials=feed2toot_usercred.txt
         client_credentials=feed2toot_clientcred.txt
+        ; Default visibility is public, but you can override it
+        toot_visibility=unlisted
 
         [cache]
         cachefile=cache.db