Need a way to choose enabled events locally

[dgw]

By default, this plugin enables all events, which can be spammy for active projects. Even projects with minimal activity might not want all supported events sent to IRC.

For example, we've configured Sopel's own webhook to send mostly issue- or PR-related events, and ignore stars/pushes.

Right now, the only way to disable certain events is to edit the webhook settings on GitHub. Eventually, we'd like to support filtering events on the bot end, to simplify the setup process (no need to open up GitHub after enabling the webhook). The biggest challenge will be designing the commands ("UI") to manage this, especially since it'll be done on a per-pair (repo:channel) basis—it's not likely to be something we can easily put in the config wizard.

Implementing this ourselves would also permit more granular filtering than GitHub supports, e.g. receiving issue open/close events but not labels/milestones/edits.

[dgw]

Hand-in-hand with #53 and the webhook handler rewrite I'm planning.