feat: Add options for which directories to filter & group to

[petemounce]

Expected Behavior

git-chglog ... --directories foo/bar/baz a/b/c

would

• filter the git log to just the entries that happened within the given directories
• uniquify the resulting set of entries

(I don't mind about a changelog entry appearing in more than one group's worth)

I work within a monorepo (with bazel), so we want to generate changelogs for just particular parts when we release those particular parts. I'd want to use bazel query to find all directories that a thing-being-released plus all it depends on, then generate a changelog for that list.

Actual Behavior

(Feature doesn't exist)

Steps to Reproduce (including precondition)

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Screenshot on This Problem (if possible)

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Your Environment

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