feat(metrics): move metrics to it's own package

[nicojs]

Move the calculation of the metrics to it's own package.
This way we can reuse it in the console reporter and the
stryker dashboard.

[nicojs]

Our integration tests are a thing of beauty 😎. I've got high confidence even when changing around a lot like in this PR.

[hugo-vrijswijk]

If I understand correctly, the plan is to have multiple language implementations for mutation-testing-metrics (which I think is a great idea, to have some sort of standard). Shouldn't the npm package be under a subfolder in packages/mutation-testing-metrics? So a node, jvm, .net, etc folder?

[nicojs]

We can add that once we want to create it, right? No need to have that now. The npm package content would remain the same.