Add support for generating coverage reports

[jwage]

We will need the ability to generate coverage reports for unit and functional tests and the ability to merge many reports to one.

[tomzx]

@jwage You may find https://github.com/andreas-weber/php-junit-merge and https://github.com/sebastianbergmann/phpcov relevant for this feature.

[jwage]

@tomzx great. thanks!

[jwage]

Allowed phpunit options to be passed through here 568fa05

phpchunkit --phpunit-opt="--coverage-html /path/to/save/coverage"

I need to think more about how to actually make it useful when running multiple chunks.

[Jean85]

I'm working on something similar in my own facile-it/paraunit, the coverage feature is nearly complete. Ask away if you need anything!