Implement "..." as a real Remarkup rule

Summary:
See https://discourse.phabricator-community.org/t/firehose-webhook-not-working-with-self-hosted-requestbin-instance/2240/. I want to make it easier to link to configuration from system text.

We currently use this weird hack with {{...}} that only works within Config itself. Instead, use @{config:...}, which is already used by Diviner for @{class:...}, etc., so it shouldn't conflict with anything.

Test Plan: Viewed config options, clicked links to other config options.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19928