allow to require signed commits

## Expected Behavior
A commit should be rejected if commitlint is configured for requiring a signed commit.

## Current Behavior
There is no option to require signed commits.

## Affected packages
* [ ] cli
* [x] core
* [ ] prompt
* [ ] config-angular

## Context
I would like to have me and my contributors to require sign git commits.
There is support for this in github. And in the github project settings one could configure that signed commits are required on PRs, but if just refuses to merge a PR instead of refuse to accept a commit.
Failing more early would simplify the process.

Start in the core package and trace how commitlint configuration is loaded and how commits are currently accepted or rejected. Compare the requested early rejection with GitHub's signed-commit requirement; done means a configured signed-commit requirement rejects unsigned commits while normal behavior remains unchanged.

Written by the indexing model from the issue text.