Add support for minimum coverage

[campezzi]

This pull request adds support for a new option in the coveralls.json file inside the coverage_options key: minimum_coverage (defaults to zero). When the user adds a value in this option, running mix coveralls or mix coveralls.html will exit with a status code of 1 if total coverage falls below that threshold. This is useful for stopping CI pipelines with tight coverage requirements.

I noticed the Html module had some totalization of stats in it, so those were moved into Stats. That has significantly cleaned up Html. There are many calculations for individual files still going on in the Local module, but I chose not to refactor those for now. We could tackle that on a later PR.

Let me know what you think! Cheers.

[coveralls]

Coverage increased (+0.6%) to 91.103% when pulling c2467b9 on campezzi:master into 05b4c1a on parroty:master.

[coveralls]

Coverage increased (+0.6%) to 91.103% when pulling b318ac3 on campezzi:master into 05b4c1a on parroty:master.

[coveralls]

Coverage increased (+0.7%) to 91.135% when pulling 0d89335 on campezzi:master into 05b4c1a on parroty:master.

[coveralls]

Coverage increased (+0.7%) to 91.135% when pulling eb11fce on campezzi:master into 05b4c1a on parroty:master.

[coveralls]

Coverage increased (+0.7%) to 91.135% when pulling 0a54cd7 on campezzi:master into 05b4c1a on parroty:master.

[coveralls]

Coverage increased (+0.7%) to 91.135% when pulling 090c960 on campezzi:master into 05b4c1a on parroty:master.

[parroty]

Thanks for the extensive enhancement and refactoring!