Allow to ignore certain packages from coverage report

[ktoso]

We generate a bit of source code during the compile phase, and don't want to check for their coverage.

In an example like:

com.banana.things
com.banana.things.gen

Story: I'd like to be able to ignore the gen package.

[Johnlon]

See #68
This is working for me but uses file names.

However if you look at the class CoverageFilter and how it's used then you may be able to figure something out.
I gave up on filtering packages because I found it difficult to make the report meaningful/accurate when one file contains multiple packages.