Add support for optional exclamation mark (!)

[EndzeitBegins]

As described in in an issue in the upstream repository, according to conventional commits, a breaking change can also be denoted by adding an exclamation mark (!) after type / scope.
This improves the visibility of breaking changes by making them visible in the commit message header.

fix(api)!: omit keys of undefined elements in JSON responses

BREAKING CHANGE: < reason here >

I'll open a PR which adds an option to enable this behaviour.
A code review would be highly appreciated.

Thanks in advance for the great work so far!