Add a bin/config set <key> --stdin < value.json flag to make CLI configuration of complex values easier

Summary:
Depends on D19003. Ref T12677. Ref T13053. For the first time, we're requiring CLI configuration of a complex value (not just a string, integer, bool, etc) to do something fairly standard (send mail).

Users sometimes have very reasonable difficulty figuring out how to ./bin/config set key <some big JSON mess>. Provide an easy way to handle this and make sure it gets appropriate callouts in the documentation.

(Also, hide the cluster.mailers value rather than just locking it, since it may have API keys or SMTP passwords.)

Test Plan: Read documentation, used old and new flags to set configuration.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13053, T12677

Differential Revision: https://secure.phabricator.com/D19004