Keep non-conforming commits as undefined type

[mdvorak]

Discussion for #1326

Conventional commits are great, but people make mistakes. Commits without conventional format got thrown away completely. This patch allows them to be displayed in Release Notes, and passed to a custom Release Notes generator as well. That way, information from those commits won't be lost during release.

Use-cases

• Enable undefined type and simply append non-conforming commits to Release Notes
• Support "dirty" repo not following conventional commits spec completely (this can happen easily when for example forking repo and updating from upstream)
• Custom Release Notes generator, which might need these commits to work with. This can be accomplished also by passing raw commit log to ChangelogNotes, which might be better for this use-case.

Since it adds a new (by default) hidden type, change should not be breaking.

[chingor13]

I think it's reasonable to parse non-conforming commits and let the downstream handlers (like changelog generators) decide whether or not to exclude them.

Implementation-wise, let's separate out the parsing of type-less commits from any new features that might use them (like a new custom changelog generator).

[mdvorak]

So what should be next steps? I'm willing to rework the PR as needed.