Fix and improve help for --cov-report

[blueyed]

--cov-report= to not output any error is hard to guess.

[ionelmc]

Neat argparse trick! But maybe we should make a more explicit --cov-report=none option? What do you think?

[blueyed]

Neat argparse trick!

It is mentioned in the README, too.

--cov-report=none sounds good.
I can image most people are using it only to generate the .coverage file, e.g. on Travis, and then use other services on top. But making none the default is probably a bit too much?!

[ionelmc]

Well I was just asking for opinion. I'm going to merge this meanwhile.