Proposal : Add block-on-denounce opt-in flag to manage actions

I would like to work on the issue #86 created by @mitchellh. From my understanding of the repo structure, the change belongs in the vouch/github.nu , where a successful denounce would call GitHub's block API to block the target user. Conversely, a successful vouch/unvouch action will call the unblock API. I am thinking about adding a opt-in flag to control if blocking happens and expose it in manage-by-issue/action.yml and manage-by-discussion/action.yml files. If this sounds good or requires any change, let me know.