fixes #448 (re #462)

This pr allows project maintainers to enforce Commitizen generated commit messages as part of the workflow triggered by the `git commit` command. 

* implements the `--hook` flag, which directs Commitizen to edit the `.git/COMMIT_EDITMSG` file directly. 
* documents the use of the `--hook` flag in the `README`, both through traditional `git hooks` and `husky`.