Add code coverage analysis functionality to `qc'

- Use `cover' with QuickCheck testing
- Reuse the `cover_*' rebar.config options
- Refactor cover-related code to separate module (`qc_cover_utils')
  for use with both `eunit' and `qc'
rebar_eunit: disambiguate internal naming

Partially revert naming changes introduced in 93689703c1:
CoverageModules -> FilteredModules
get_coverage_modules -> get_matching_modules

Having the name "coverage" meaning "filtered/selected modules" can be
confused with code coverage.
Wrap cover:stop() call as rebar_cover_utils:exit/0