Add settings to explicitly exclude folders from changelogbot check

[jbohanon]

In an effort to clean up our changelog directories, @gunnar-solo has come up with a script to condense supported LTS versions' changelogs into minor-version directories, and everything older into an _archive directory. In order for Lord Changelog-bot to accomodate this change, some way of informing it that there are directories that are okay to skip needs to be introduced. In this PR, we introduce a new setting into the validation.yaml file to enumerate which directories should be excluded from consideration/erroring. An example validation.yaml might look as so:

ignoredDirectories:
- _archive
- 1.9
- 1.10
- 1.11
- 1.12

It remains to be discussed whether this should be done on an explicit basis or via some sort of pattern matching.