#640 - Exit zero status based on severity level

[maciejstromich]

This PR enables more fine grained control over exit zero status based on severity level.

[maciejstromich]

@ericwb @ghugo @sigmavirus24 @lukehinds the pep8 check is failing with bunch of

tests/unit/formatters/test_html.py:10:1: H216: The unittest.mock module should be used rather than the third party mock package unless actually needed. If so, disable the H216 check in hacking config and ensure mock is declared in the project's requirements.
import mock

how do you want to approach this? should this be a separate PR or can I include it in the current one?

[maciejstromich]

ah apparently @ericwb created already a PR for that #685

[maciejstromich]

@zealotous thanks a review.