Allow for defining command callbacks, for issue/PR comments containing @ the github app

[joshuadavidthomas]

A nice to have feature would be the ability to @ reply the github app that django-github-app is installed on with a command and have the event router handle that appropriately. I'm thinking about how you can @ Dependabot to rebuild or recreate a PR, I can see how that would be super useful to have built-in to the library -- as opposed to someone building out all of that on their own.

Part of the reason I wrapped gidgethub.routing.Router and providing our own event method for registering webhook handlers in the first place was to support extending in a way that would provide a seamless API.

from django_github_app.routing import GitHubRouter

gh = GitHubRouter()

@gh.command("recreate")
async def recreate_pr(event, gh, *args, **kwargs): ...

So someone commenting on a PR with @user-github-app recreate would trigger this callback.

Could also see use in being able to scope it to either PR or issues as well:

from django_github_app.routing import GitHubRouter

gh = GitHubRouter()

@gh.command("recreate", scope="pr")
async def recreate_pr(event, gh, *args, **kwargs): ...

@gh.command("recreate", scope="issue")
async def recreate_issue(event, gh, *args, **kwargs): ...

Unsure about that wording of scope, but settling on that can come later.