Allow to compute coverage only for public methods/fields

[stieglma]

I have a rather large codebase and want to make assumptions about the javadoc-coverage only from the public (and therefore accessible from everywhere) part of the code. It would be ok for me to send you a merge request for that, but at first I wanted to know if you like the idea of having this option, too?

[manoelcampos]

I think it's a great contribution. I'm looking forward to your PR. Thanks.