[UX Demo] Cog stubbed plugin

Description

Create a plugin for Cog that responds to realistic commands, but just stubs out the backend, for demo purposes only.

Proposal

Convention would use a command structure like !gitlab:noun verb [object]. The gitlab part is optional if the noun is unique amongst all installed plugins.

Potential vocabulary:

• !gitlab:project create
• !gitlab:issue create [from last N]
• !gitlab:issue add [from PERSON]
• !gitlab:issue open [ISSUE]
• !gitlab:deploy staging production
• !gitlab:deploy v8.10.0 production
• !gitlab:deploy BRANCH ENVIRONMENT
• !gitlab:rollback [ENVIRONMENT]

Links / references

Meta issue: #19793 (closed) "Real" issue: omnibus-gitlab#1412 (closed)

/cc @northrup