support custom function in conventional changelog spec

[jedwards1211]

Yet another example of static config formats giving us hell.

I want the following rule for changelogs for my monorepo subpackages (this appears to be the convention this monorepo follows itself?)

• commits that include the package name in the scope should be included
  (this includes comma-separated scope lists like fix(package-a,package-b): blahblah)
• commits that have no scope should be included

If I could just pass a custom function that takes the commit and returns the changelog section and whether it's hidden, then implementing this logic would be easy.

Thanks to the static config, this is currently impossible.