bounce: use new boolean config options

* update docs for better flow.
* rename [checks] -> [check], so it's consistent among plugins that have it.

% ./run_tests tests/plugins/bounce.js
Running tests:  [ 'tests/plugins/bounce.js' ]

bounce.js
✔ refresh_config - yes
✔ reject_all - disabled
✔ reject_all - not bounce ok
✔ reject_all - bounce rejected
✔ empty_return_path - none
✔ empty_return_path - has
✔ single_recipient - valid
✔ single_recipient - invalid
✔ single_recipient - test@example.com
✔ single_recipient - test@example.com,test2@example.com
✔ bad_rcpt - test@good.com
✔ bad_rcpt - test@bad1.com
✔ bad_rcpt - test@bad1.com, test@bad2.com
✔ bad_rcpt - test@good.com, test@bad2.com
✔ has_null_sender - <>
✔ has_null_sender -  
✔ has_null_sender - user@example
✔ has_null_sender - user@example.com

OK: 20 assertions (109ms)