Ignoring certain events

For example in a configuration file:

ignore:
    issues:
        - labeled
    pull_request:
        - labeled
        - assigned

would prevent sending notifications for the labeling of issues and for the labeling and assigning of PRs.

[ulfsauer0815]

There are currently some implicit rules about dropping events implemented that made sense to me, because the event stream can easily become spammy.

Labeling and the assignment of pull requests/issues is completely ignored at the moment.

These rules don't necessarily make sense in every environment. They might be too restrictive for a small projects like this one and not restrictive enough for a very active one.

Making this configurable would be nice and is easier to implement once file-based configuration is done (part of #16).

I'm not sure how flexible this needs to be.
Only checking the action, or checking for presence of any arbitrary nested value, e.g. commits[].author.name='CI Bot'. The latter would be quite complex.