Block commit when changes aren't staged

[xipasduarte]

Is your feature request related to a problem? Please describe.
After starting a commit, which triggers the linting process, we get changed files (if any fall out of the PHPCS specs). After this, if we want, we can commit again and the process doesn't identify that the files are staged, which generates a commit without clean code.

Describe the solution you'd like
The package should block any commits while the staged files have issues. This means we should lint the files that are about to be committed and leave the other alone.
To allow users the chance to skip the linting step we should rely on a flag when committing.