Suggestion: Support include/exclude for topics

[anderssonjohan]

Prerequisites:

• Is the functionality available in the GitHub UI? If so, please provide a link to information about the feature.

• Is the functionality available through the GitHub API? If the functionality is available, please provide links to the
  API documentation (https://developer.github.com/v3/) as well as the Octokit documentation (https://octokit.github.io/).

• If the functionality is not yet available in the API, it would be helpful if you
  contacted support (https://support.github.com/) or posted in the Community Forum (https://github.community/). Please
  include a link to the forum post if you create one or a copy of the response from support.

New Feature

Please describe the desired new functionality:

Originally posted by @martinm82 in #408 (comment)

Add support for alternative topics configuration where topics matching a given pattern can be excluded from delete operations. This allows other tools to create topics without having them deleted by safe-settings.

Example: The following config would keep the topic team-unicorn created by another tool or user.

topics:
  include:
    - probot
  exclude:
    - name: ^team-

Include can be used without specifying exclude in the config, while also supporting the current format:

topics:
  - probot

[wwalters12]

Would love to see this feature!

We would like to set topics for each suborg without deleting some topics that already exist. We know this could be achieved by having repo level settings, but the amount of repos that would need to be specified make that impractical.

[martinm82]

Also worth to mention is that with this feature one could define on org level to not delete any non-tracked topic, like for labels: #408 (comment)