Code coverage aided mutations.

[xd009642]

So this may be worth creating an issue for, but this is largely an idle thought I had yesterday. Mutation testing improves on things like coverage by making sure the tests are actually useful and not just hitting a ton of lines/conditions but not checking any of the values. Now if we already have code coverage results for our tests and can see some functions aren't tested at all we could save time by not applying mutations to them - after all none of the mutations would be caught.

For maximum usability this should probably take the form of accepting an optional argument of some open coverage format like an lcov report or cobertura.xml which a number of coverage tools already output.