Allow grouping by labels

[embik]

Since https://github.com/kubermatic/dashboard now has multiple components, it would nice if gchl could split a generated changelog by labels, e.g. when grouping by sig/api and sig/ui, it will filter PRs based on that and create two separate sections that hold all changes (while still maintaining the existing structure of grouping by kind labels below that) for the respective part/module of the repository.