Allow pre-commit adapter to use custom actions

Summary: Looks like I missed this when implementing custom actions and hence you can't currently use custom actions on the pre-commit adapters.

Test Plan: Added a custom action to a pre-commit Herald rule.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10316