Introduce a new option to ensure that the subject respects certain “case” rules.

[tbouffard]

This is inspired by what is provided by @commitlint/config-conventional

In my repositories, I prefer that the subject starts by a lower case letter. This could be enforced by a rule like https://github.com/conventional-changelog/commitlint/tree/master/%40commitlint/config-conventional#subject-case

Notice that @commitlint/config-conventional provides a lot of other features that we could add to this action as well. For example

• header-max-length
• subject-full-stop

What do you think about this @jef?