Add topics and exclude.topics to GitHub & GitLab config

[brendan-kellam]

This PR adds the ability to filter what repos are indexed based on the topics that do/don't exist on a given repo. E.g.,:

{
    "$schema": "./schemas/v2/index.json",
    "repos": [
        {
            "type": "github",
            "orgs": [
                "sourcebot-dev"
            ],
            // Include repositories in `sourcebot-dev` that have at least one of these topics.
            "topics": [
                "hacktoberfest",
                "advent_of_code"
            ],
            "exclude": {
                // Explicitly exclude a topic from being indexed.
                "topics": [
                    "TypeScript"
                ]
            }
        }
    }
}

In the above case, a repo with the label hacktoberfest will be indexed, but a repo with both hacktoberfest and TypeScript will not be indexed.

Fixes #107