Add support for optional exclamation mark (!)

According to [conventional commits](https://www.conventionalcommits.org), 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 >
```

We started to take a look at Commitizen in combination with `@digitalroute/cz-conventional-changelog-for-jira`. Instead of contributing the feature there, I thought it would be more effective to contribute this feature "upstream" instead, as it has nothing to do with Jira.

Any thoughts about this? Are PRs welcome?
Thanks in advance for the great work so far!